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Struvite Production at Commercial Dairies with Use of a Mobile System and Comparisons to Alternative Nutrient Recovery Systems
Joseph. H. Harrison1,*, Kevin Fullerton1, Elizabeth Whitefield1, Keith Bowers2, Clinton Church3, Matias Vanotti4, Patrick Dube4
Published in Applied Engineering in Agriculture 38(2): 361-373 (doi: 10.13031/aea.14836). 2022 American Society of Agricultural and Biological Engineers.
1 Biological Systems Engineering, Washington State University, Pullman, Washington, USA.
2 Ostara, Seattle, Washington, USA.
3 Pasture Systems and Watershed Management Unit, USDA-ARS, State College, Pennsylvania, USA.
4 Coastal Plains Soil, Water and Plant Research Center, USDA-ARS, Florence, South Carolina, USA.
* Correspondence: jhharrison@wsu.edu
The authors have paid for open access for this article. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License https://creative commons.org/licenses/by-nc-nd/4.0/
Submitted for review on 6 September 2021 as manuscript number PAFS 14836; approved for publication as a Research Article by the Plant, Animal, & Facility Systems Community of ASABE on 30 January 2022.
Mention of company or trade names is for description only and does not imply endorsement by the USDA. The USDA is an equal opportunity provider and employer.
Highlights
Abstract. Effective methods for capture of phosphorus (P) from dairy manure are needed to promote recycling of P. A mobile system consisting of a fluidized bed technology with a 3,200 L cone mounted on an 8-m trailer was used to evaluate the variability in capture of P from liquid dairy manure in the form of struvite. Batches of 13,000 L of manure were pre-treated with sulfuric acid or oxalic acid, and magnesium chloride prior to pumping through the cone at a flow rate of ~20 to 28 L min-1. Sodium hydroxide or aqueous ammonia were pumped into the base of the cone to raise the pH and form struvite. On many occasions the system did not work well and negative ortho-P removals were observed. When the struvite system was working well, ortho-P capture using dairy manure ranged from 1% to 76%, averaging 34% over the course of 7 to 9 h of operation. Ortho-P capture was highest at 84%, and total P capture of 62% using anaerobically digested manure with pH modifiers of oxalic and/or sulfuric acid and aqueous ammonia. Major factors and concentrations that promote formation of struvite were: total suspended solids (<10,000 mg L-1), Ca (<600 mg L-1), Fe (<25 mg L-1), and ortho-P (>50 mg L-1). Partial budget economic analysis indicated that number of lactating cows and cropland were critical factors for achieving whole farm P balance. A comparison to two other technologies for capturing P from manure indicated the three systems provide an initial end product for off-farm export in the range of $0.003 to $0.02 per L of manure based on a partial budget analysis considering the variable cost of chemicals.
Capturing P from livestock manure in the form of struvite (magnesium-ammonium phosphate - MgNH4PO4.6(H2O)) has been proposed as a method for removing excess P for export from the farm and to subsequently be used as a P source for crop production (Bowers and Westerman, 2005; Demirer and Yilmazel, 2013, Tao et al., 2016). Hilt et al. (2016) reported the value of dairy manure-derived struvite as a P source for alfalfa, corn silage, triticale, and oats. Martín-Hernández et al. (2018) concluded that production of struvite to be one of the most cost-effective methods for capturing nutrients from cattle manure that has been anaerobically digested. In addition, it has been suggested that production of struvite from livestock manure could avoid an estimated 22% to 36% of P losses from agriculture at a national level (Martín-Hernández et al., 2020). Prior work has indicated poor (10% to 15%) struvite recovery from raw dairy manure due to high concentrations of calcium (>600 mg L-1) ions in the manure that react with P to form calcium phosphates (Harris et al., 2008; Qureshi et al., 2008; Zhang et al., 2010; Huchzermeier and Tao, 2012). It is necessary to decrease the pH of the manure to <6 to break the Ca-P bond. Raising the pH (>pH of 6) to form struvite can result in re-combination of the calcium and phosphate ions. Two factors may explain why the re-combination does not fully occur. First, the increased sulfate ion concentration resulting from the use of sulfuric acid (hydrogen sulfate) to depress pH and dissolve calcium sulfate may occupy some of the calcium ions, which exist in high concentration in dairy wastewater and interfere with the removal of phosphate as struvite, by forming a calcium-sulfate complex. Elgquist and Wedborg (1979) proposed a stability constant of 25.4 for this complex in seawater. A stability constant of 25.4 for this complex indicates that, at equilibrium, the effective concentration of the calcium sulfate complex will be 25.4 times the product of those of the constituent ions (calcium and sulfate). To satisfy this condition, a significant portion of the calcium ions will be tied up in the complex and thus unavailable to interfere with struvite precipitation. Second, in most of the cases, the pH is raised to a value lower than that of the starting raw wastewater, possibly leading to a lower tendency for calcium ions to out-compete magnesium and ammonium ions in combining with phosphate. Secondly, it has been reported that a Ca2+:Mg2+ molar ratio greater than 0.2 hinders struvite formation (Tao et al., 2016). A Ca2+:PO43- molar ratio greater than 0.5 to 1.0 inhibits struvite formation and purity (Tao et al., 2016).
Recovery of P as struvite has been shown to be as high as 80% to 98% in swine manure and poultry litter (Bowers and Westerman 2005; Demirer and Yilmazel, 2013). Factors that inhibit struvite formation are high ionic strength and alkalinity of the manure, presence of competing ions, suspended solids, and the low ratio of ortho-Phosphate to total phosphorus (TP) (Schuiling and Andrade, 1999; Zeng and Li, 2006; Moerman et al., 2009; Shen et al., 2011; Zhang et al., 2010; Huchzermeier and Tao, 2012). Siciliano et al. (2020) have provided a comprehensive review of advances in struvite precipitation technologies that focused on factors that affect the removal and recovery of nutrients from aqueous waste and wastewater. Processing of dairy manure through anaerobic digestion converts organic phosphates into inorganic phosphorus, which increases the amount of available phosphate for capture in struvite (Ma et al., 2017). However, phosphate can alternatively be precipitated as calcium and magnesium phosphates (Güngor and Karthikeyan, 2008). One potential solution to this challenge is the addition of oxalic acid since it not only reduces the pH but also binds to Ca leaving ortho-P available for struvite formation (Brown et al., 2018).
While struvite has been suggested as one of the most cost-effective methods to capture P from livestock manure (Martín-Hernández et al., 2018), two alternative approaches to capture of P from livestock manure are the production of newberyite (New-P, MgHPO4.3H2O; Vanotti et al., 2018) or capture of P in form of a precipitant (Church et al., 2016, 2017, 2018a, 2018b, 2020). The newberyite recovery of P via Mg precipitation was enhanced by combining it with ammonia (NH3) recovery through gas-permeable membranes (GPM). The combined New-P process (Vanotti et al., 2018) applied to digested swine manure provided 97% to 98% N recovery in one product and complete (~100%) P recovery in another product. The preliminary removal of the soluble ammonia destroys carbonate alkalinity. As a result, the phosphates produced contain high-grade P2O5 (37% to 46%) and low N, similar to the composition of the bio-mineral newberyite (Vanotti et al., 2017, 2018). The MAnure PHosphorus EXtraction (MAPHEX) System (Church et al. 2016, 2017, 2018a, 2018b, 2020) relies on sorption of dissolved P to solid particles either added to or formed in the liquid manure, then removing those solids by solid-liquid separation, while most of the nitrogen (greater than 90%) is retained in the liquid for beneficial use by the farmer. The MAPHEX System has been shown to remove greater than 95% of Total P from dairy manures (Church et al., 2016, 2017, 2018a), and P removal efficiencies on swine manure were greater than 96% (Church et al., 2020). P removed by the system is contained in low-P bulk solids that are often composted and used for bedding by the farmer on dairy farms, and high-P solids that can be more economically transport for greater distances to lands that are in need of P.
Struvite production from dairy manure have been reported for individual operations at pilot scale (Schuiling and Andrade 1999; Bowers et al., 2007; Zhang et al., 2015). No reports are available characterizing the effectiveness of struvite production that might be experienced with dairy manure under varying field conditions at commercial dairies. Therefore the primary objective of this work was to characterize the variability of capture of P as struvite from liquid dairy manure from multiple sources of manure from dairies operated under commercial conditions, including manure that had been anaerobically digested. This was accomplished at pilot scale with use of a trailer-mounted mobile struvite production system (MSPS). Since adoption of manure management technologies are dependent on their technical effectiveness as well as their economic feasibility, a second objective was to make an economic comparison of the struvite system to two alternate systems reported by Church et al. (MAnure PHosporus EXtraction (MAPHEX) System) (2016, 2017, 2018a, 2018b, 2020) and Vanotti et al. (Newberyite-P) (2018).
Materials and Methods
Mobile System
The MSPS utilized a fluidized bed technology with upflow design (Bowers and Westermann 2005b) with a 3,200 L cone mounted on an 8-m trailer (fig. 1) (Daritech, Inc., Lynden, Wash.) allowing easy transport from farm to farm. The overall length or height of the cone was 5.5 m. The outside diameter at the bottom of the cone was 14.3 cm and increased steadily for 184 cm of the cone length to an outside diameter of 368 cm, forming the primary reaction section of the cone. The diameter then increased from 368 to 481 cm diameter in the next 113 cm of cone length. This section served as a point for settling of struvite crystals. The remaining 56 cm of cone length remained the same at 481 cm in diameter. Liquid manure (liquid-solids separated) was pre-treated in ~17,000 L polytanks with manual addition of 93% to 98% sulfuric acid (avg. of 43 L) (Northstar Chemical, Inc. Sherwood, Ore.) or oxalic acid (33 to 125 kg) (oxalic acid dehydrate - Phibrochem, Teaneck, N.J.) to lower the pH to ~5.0 to 5.4 to release the bond between Ca and P. Magnesium chloride (avg of 51 kg) (flakes - Compass Minerals International, Overland Park, Kan.; 30% solution - Cascade Columbia Distribution Company, Seattle, Wash.) was also added (1.5× the Ca or ortho-P concentration) to the pre-treatment tank to allow for sufficient concentration of Mg for formation of struvite. After addition of acid and magnesium chloride to the manure, the contents were mixed for ~1 h until the desired pH (measured with portable pH meter) was achieved, then the pre-treated manure was allowed to settle for 24 to 48 h before being transferred to the reaction cone. The amount of pre-treated manure that was pumped through the cone was less than the total amount pre-treated due to settling of solids in the pre-treatment tank. Batches of 13,000 L of manure were evaluated and the system was operated at a flow rate of ~20 to 28 L min-1. The cone was seeded with ~23 kg of struvite crystals for establishment of a surface for subsequent crystal formation. Sodium hydroxide (avg of 30 kg) (50% - Wesmar Co., Lynnwood, Wash.) or 20% aqueous ammonia (avg of 66 L) (McGregor Co., Ritzville, Wash.) were pumped into the base of the cone to raise the pH to ~7.0 to 7.5. Typical runs were 6 to 12 h in duration with samples obtained at regular intervals to assess capture of P as struvite. After each run, the fluidized bed containing the original bed and newly formed struvite was allowed to settle for 12 to 24 h prior to draining from the bottom of the cone, and subsequently allowed to air dry prior to sampling for nutrient analyses.
 |
| Figure 1. Picture of mobile struvite system. |
Study 1. Thirty-two farms
Study 1 was conducted to evaluate the performance of the MSPS when using sulfuric acid and NaOH as pH modifiers.Farms in Washington State were selected based on their ability to meet the following criteria: access to manure that had been processed with solid-liquid separation, manure was easily accessible for transfer into the pre-treatment tank, and manure had an initial chemical profile for total P, ortho-P, Mg, Ca, and solids that indicated struvite formation would be successful. It was desirable to use liquid manure that had >50 mg L-1 ortho-P since lower concentrations of ortho-P would suggest that a given farm did not need to capture excess P for export off-farm. Liquid manure with less than 2.0% solids were considered for evaluation with the system since large particle solids and suspended solids impede the formation of struvite. It was desirable to have Ca at one-third of ortho-P or the need for additional acid and Mg would be required to minimize the interference in struvite formation by Ca (Le Corre et al., 2005; Zhang et al., 2010; Tao et al., 2016). It was desirable to have Fe as low as possible since each mg L-1 of Fe can bind 0.5 mg L-1 of ortho-P. Two of the 32 farms had operating anaerobic digesters. Samples of manure were obtained prior to pre-treatment, after pre-treatment, and at ~4 and 8 h after the process run was initiated. In addition, the composition of the struvite produced was analyzed. A recording of pH was obtained at ~hourly intervals at the mid-point and top of the cone.
Study 2. Alternative pH modifiers
Study 2 was conducted to evaluate the hourly performance of the MSPS as affected by source of manure (anaerobically digested (AD) manure or non-AD manure); use of pH modifiers of oxalic acid, sulfuric acid, NaOH, ammonia water; and starter bed source. Batches of 9,500 to 11,000 L of manure (non-AD and AD) from a single farm were pre-treated with an acid pH modifier (oxalic acid or sulfuric) and subsequently pumped into the mobile struvite system with addition of a base pH modifier (NaOH or aqueous ammonia). The two sources of bed were fine granular (0.01 to 0.05 mm) and spherical (0.35 mm). Samples of manure were obtained prior to pre-treatment, after pre-treatment, and hourly after the process run was initiated. A recording of pH was obtained at ~hourly intervals at the midpoint and top of the cone.
Sampling and Analyses
Samples were designated as raw (prior to any addition of chemicals) or instantaneous (samples obtained at a given point in time during the process run). Samples obtained at approximately the mid-point of a run were called mid-stage, and samples obtained at the end of a run were called final stage. Mid-stage and final stage samples were obtained from the overflow point at the top of cone (see fig. 1). Raw manure samples were analyzed for ortho-P, total- P, Ca, Fe, Mg, NH4-N, total suspended solids, and total solids (table 1). Pre-treated samples (after addition of acid and magnesium chloride) and samples during the process run were analyzed for ortho-P and total-P. Samples of the struvite bed were analyzed for ortho-P, total-P, citrate soluble phosphoric acid (AOAC 2012), TKN, Mg, Ca, Fe, ash, and neutral detergent fiber (NDF). Sample analyses were conducted by one of four laboratories: Edge Analytical – Burlington, Washington; Custom Dairy Services – Lynden, Washington; and Soil Test Farm Consultants – Moses Lake, Washington; US Ag Analytical Services – Pasco, Washington. Any sample analyzed for ortho-P was acidified to pH 4.5 to ensure that suspended calcium phosphates were dissolved to release ortho-P into solution before filtering to remove suspended solids. The methods of analysis for TKN, P, Mg, Ca, Fe, ash ammonia-N, and ash were the standard methods for examination of water and wastewater (SM) 4500-Norg C (Rev#, 2.2), SM 4500-NH3H (Rev#, 2.3), SM 4500-P F (Rev#, 1.1), SM 2540 G (Rev#, 2.0) (Greenberg, 1992), and US EPA approved methods 6010 B ICP-OES (Inductively Coupled Plasma-Optical Emissions Spectroscopy) (Harris et al., 1984). The analysis of NDF was by Mertens (2002) and total suspended solids as described by Fishman and Friedman (1989).
| Table 1. Summary of composition of raw manure samples from 32 dairies – study 1. | ||||||
| Parameter (mg L-1) | Average | SE | SD | Min | Max | n |
| Ortho - P | 217 | 30 | 154 | 56 | 802 | 26 |
| Total P | 669 | 196 | 833 | 92 | 2,810 | 19 |
| Ca | 707 | 88 | 625 | 185 | 4,145 | 51 |
| Fe | 158 | 36 | 175 | 7 | 658 | 24 |
| Mg | 447 | 63 | 413 | 100 | 2301 | 43 |
| NH4-N | 1,227 | 258 | 929 | 399 | 2843 | 13 |
| Total suspended solids | 18,971 | 3,227 | 21,158 | 200 | 83,400 | 43 |
| Total solids | 18,189 | 1,616 | 9827 | 4,400 | 3,9000 | 37 |
Economic Analyses
A partial budget analysis was conducted to evaluate the variable cost (i.e, chemicals) to capture excess P as struvite and compare it to two other experimentally tested technologies. A partial budget analysis was chosen since each of the three technologies are relatively new and their operation under full-scale farm conditions has yet to be experienced and therefore no data exists from which to conduct a more comprehensive economic comparison amongst the systems. Excess P based on amount used by crops was determined based on a standardized farm with the following assumptions: 41 kg [milk] cow-1 d-1, 67 g[excreted P] d-1, 48 g[struvite formed] cow-1 d-1 (Nennich et al., 2005), 27 kg crop uptake per year, and varying lactating cows from 600 to 1000, or varying crop acres from 243 to 362 ha. Costs of chemicals were priced at wholesale: sulfuric acid (93%) - $0.60/L; oxalic acid (oxalic acid dihydrate, 60-100%) - $0.14/kg; liquid NaOH (50%) - $0.59/L; MgCl (100%) - $0.68/kg: and aqua-ammonia (20%) - $0.05/L. While not included in the economic evaluation, the cost per day for equipment, maintenance, and labor was estimated at $0.085 cow-1 d-1. Equipment (initial construction and replacement parts during maintenance) for a 1,500-head dairy was estimated at $262,000 over a period of 15 years, equating to 3.2 cents per cow per day. Labor (operating and maintenance) was estimated at four hours per day at a cost of $20 per hour, equating to 5.3 cents per cow per day, bringing the total for equipment and maintenance (including labor) to 8.5 cents per cow per day.
Data Analyses
This research was designed as a field case-study and therefore simple means and standard errors were calculated on most data. Mean comparisons (Proc GLM using SAS v. 9.4 for Windows (SAS Institute Corp., Cary, N.C.) were made in study 1 on the reduction in concentration in ortho-P or total P, or% reduction in ortho-P or total P.
Results and Discussion
A summary of the analyses of raw manure collected in study 1 prior to use with the MSPS are shown in table 1. The average ortho-P and total P concentrations were 217 and 669 mg L-1, respectively. A minimum of 50 mg L-1 of ortho-P is desired for successful capture of P as struvite, and the minimum ortho-P observed was 56 mg L-1. Struvite formation requires that P be in the ortho form in order to effectively create the links between ammonia-magnesium-phosphate. Sources of manure that contain a high percentage of ortho-P are swine and anaerobically digested dairy manure (Bowers and Westerman, 2005a, 2005b; Bowers et al., 2007; Brown et al., 2018). The average Ca concentration was 11,819 mg L-1, with a range of 185 to 47,600 mg L-1. Dairy manure is known to have a greater concentration of Ca compared to swine manure and thus requires additional chemical treatment for the formation of struvite (Bowers and Westerman, 2005a, 2005b). It is desirable to have the Ca concentration less than 1/3 that of ortho-P so that free P is available for struvite formation. Since this was not achieved, acid (sulfuric) was required to break the Ca-P bond and make ortho-P available for struvite formation (Zhang et al., 2010). The Fe concentration averaged 158 mg L-1, and each mg L-1 of Fe can bind 0.5 mg L-1 of ortho-P thus inhibiting struvite formation when Fe content is high. The average concentration of Mg was 447 mg L-1 and less than needed to provide for effective formation of struvite without supplemental Mg. The average NH4-N was 1,227 mg L-1 and adequate for all farms except one. The farm with low NH4-N had manure that had been stored for many months and was in a dry-arid region of the state with high temperatures (>27°C) which resulted in volatilization of N. The average suspended solids and total solids were 18,971 and 18,189 mg L-1, respectively. Solids greater than 2500 mg L-1 in dairy manure result in reduced formation of and retention of struvite crystals in the cone due to increased viscosity. In contrast, Tarrago et al. (2018) reported that with swine manure TSS of up to 3,000 mg L-1 could be beneficial to the struvite formation by acting as nuclei favoring heterogeneous nucleation.
A summary of the amount of manure treated, use of pH modifiers and addition of MgCl, manure processed through the MSPS, and pH of treated manure are shown in table 2. The average amount of manure that was treated in a run (tank) was 15,822 L and was reflective of the ~19,000 L tanks that were utilized. After treatment of the manure with acid and MgCl, the manure was allowed to settle and this resulted in an average amount of manure of 11,183 L being processed through the MSPS. Due to the Ca concentration of the initial manure it was necessary to add an average of 43 L of sulfuric acid were used (2.7 L per 1000 L of manure) to lower the pH below 6.0 to break the Ca-P bond. When oxalic acid was used to lower the pH, an average of 125 kg were used or 7.9 kg per 1000 L of manure. Oxalic acid has the advantage of lowering the pH to break the Ca-P bond and also sequestering Ca (Brown et al., 2018). The average addition of MgCl was 51 kg or 3.2 kg per 1000 L of manure to achieve adequate Mg for formation of struvite. The average amount of NaOH used to raise the pH in the cone was 30 kg or 2.7 kg per 1000 L. The average pH (~ hourly) in the mid cone section (~3 m) and top of the cone (~6 m) were 7.7 and 7.5, respectively. The pH for good crystal formation is in the range of 7.0 to 7.7 for high-calcium wastewaters. Higher pHs can result in greater formation of struvite but the need for greater amounts of pH modifier is not cost effective.
| Table 2. Summary of amount of manure treated and use of pH modifiers and cone pH – study 1. | ||||||
| Parameter | Average | SE | SD | Min | Max | n |
| Treated volume of manure, L | 15,822 | 160 | 946 | 14,060 | 17,480 | 35 |
| Sulfuric acid, L | 43 | 2.8 | 17 | 19 | 85.5 | 34 |
| MgCl, kg | 51 | 3.6 | 21 | 12.6 | 102 | 35 |
| NaOH, kg | 30 | 1.5 | 9.0 | 4.7 | 45.6 | 35 |
| Volume processed through cone, L | 11,183 | 43 | 248 | 2,450 | 3,400 | 35 |
| Mid cone pH | 7.7 | 0.1 | 0.4 | 6.7 | 8.6 | 35 |
| Top cone pH | 7.5 | 0.1 | 0.5 | 6.7 | 8.6 | 35 |
One approach we used to assess the effectiveness of struvite formation was to evaluate the instantaneous reduction (reduction at a given point in time during the process run) in the concentration or % of ortho-P or total-P for (study 1 - table 3). The data in table 3 are categorized to represent all data collected, or data from the dairies in which we saw a positive reduction in concentration of ortho-P or total-P at the mid-stage (~4 h) of the run or the end (~8 h) of the run. A positive reduction infers that net decrease in concentration of P has occurred. A negative reduction infers that a net increase in concentration has occurred P. When considering data from all farms (35 observations), the average instantaneous reduction in concentration of ortho-P at the mid-stage of process runs or the final stage of process runs was negative, with a range of values from -484 to 196. A similar negative response was noted for average % reduction of ortho-P at the mid-stage of process runs, with a range of -582 to 55. The average instantaneous % change in ortho-P at the final stage of process runs was positive (7.8%), with a range of -175 to 76. In contrast to the average changes in concentration or % of ortho-P, the average instantaneous reduction in concentration or % total-P was positive with values of 26 and 29 mg L-1 for the mid-stage and final stage of process runs, and 14% and 19% for the mid-stage and final stage of process runs, respectively. While statistical differences were not noted, these data suggest that taking samples ~8 h into a process run will provide a better estimate of system performance than samples obtained at ~4 h into a process run (more discussion in study 2).
| Table 3. Summary of instantaneous reduction in concentration or% othro-P and total P in manure at various stages of operation of the mobile struvite system - study 1 – 32 dairies. | ||||||||||||
| All Data | Ortho-P | Ortho-P | ||||||||||
| mg L-1 | n | SE | Min | Max | % | n | SE | Min | Max | |||
| Mid-stage[a] | -4[b][c] | 35 | 17.5 | -456 | 196 | - 9[b] | 35 | 18 | -582 | 55 | ||
| Final[d]stage | - 9[b] | 35 | 23.1 | -484 | 150 | 7.8[b] | 35 | 9.4 | -175 | 76 | ||
| Total-P | Total P | |||||||||||
| mg L-1 | n | SE | Min | Max | % | n | SE | Min | Max | |||
| Mid-stage[a] | 26[b] | 35 | 16 | -250 | 258 | 14[b] | 35 | 5.4 | - 63 | 63 | ||
| Final[d]stage | 29[b] | 35 | 18 | -481 | 211 | 19[b] | 35 | 6 | -121 | 72 | ||
| Positive Data | Ortho-P | Ortho-P | ||||||||||
| mg L-1 | n | SE | Min | Max | % | n | SE | Min | Max | |||
| Mid-stage | 48[b] | 21 | 8.8 | 8 | 196 | 33[b] | 21 | 3.4 | 2 | 55 | ||
| Final stage | 46[b] | 26 | 6.6 | 1 | 150 | 34[b] | 26 | 3.3 | 1 | 76 | ||
| Total-P | Total-P | |||||||||||
| mg L-1 | n | SE | Min | Max | % | n | SE | Min | Max | |||
| Mid-stage | 62[b] | 27 | 13.2 | 1 | 258 | 29[b] | 26 | 3.4 | 1 | 63 | ||
| Final stage | 58[b] | 29 | 9.2 | 2 | 211 | 29[b] | 23 | 3.0 | 0 | 72 | ||
[a] ~4 h after initiation of run. [b] Means with unlike superscripts differ, P < 0.05. [c] Means were within data category (all or positive), type of P (ortho-P or total P), and unit of evaluation (concentration or% reduction). Means with unlike superscripts are different. [d] ~8 h after initiation of run. Samples obtained at approximately the mid-point of a run were called mid-stage, and samples obtained at the end of a run were called final stage. | ||||||||||||
To provide a comparison of when the system was performing well, a summary of those farms with positive changes are shown in table 3 (21-29 farms). The average instantaneous reduction in concentration of ortho-P at the mid-stage and final stage was 48 and 46 mg L-1, respectively. Average reduction in ortho-P as a % was 33 and 34% at the mid-stage and final stage, respectively. The average instantaneous reduction in concentration or % total-P was positive with values of 62 and 58 mg L-1 for the mid-stage and final stage, and 29% for the mid-stage and final stages, respectively.
A number of factors can result in an increase in concentration of P (or negative reduction) in samples leaving the cone. Our experience suggested a primary reason was in the first few hours of operation some of the initial bed material used to seed the cone can become dissolved and result in an increase in the concentration of P (discussed in more detail in study 2).
Table 4 summarizes the composition of struvite product harvested from the MSPS after each run. The composition reflects the addition of new struvite crystals and other compounds (Ca-P, fiber) in addition to the composition of the original bed material used in each MSPS run. The composition of pure struvite is approximately 57,100 mg kg-1[N], 126,200 mg kg-1[P], and 99,000 mg kg-1[Mg]. Based on the P, TKN, or Mg concentration, data in table 4 suggest the struvite to be presumptively 81% to 85% pure. Absolute purity would have to be confirmed with scanning electron microscopy. The concentration of free ortho-P represented less than 0.6% of total P present in the samples evaluated. Citrate soluble-P is used as an indicator of the readily available P for crops and averaged 26% of total P, confirming the concept of struvite being an intermediate release-rate form of P for crop growth (Bationo et al., 1991). While processing the struvite samples for drying and subsequent chemical analyses, visual observation suggested presence of fiber in the samples. An analysis of NDF to estimate cellulose plus hemicellulose confirmed the presence of fiber, but the amount was quantitatively small (3 mg kg-1) indicating that the struvite was relatively free of fibrous material from manure.
| Table 4. Summary of composition of struvite product - study 1. | ||||
| Parameter | Average | SE | SD | n |
| Ortho-P, mg kg-1 | 644 | 72 | 406 | 32 |
| Total-P, mg kg-1 | 107,608 | 3,998 | 22,619 | 32 |
| Citrate soluble phosphoric acid, % Total P | 26 | 0.5 | 2.7 | 32 |
| TKN, mg kg-1 | 48,637 | 1,902 | 10,762 | 32 |
| Mg, mg kg-1 | 81,249 | 2,880 | 16,290 | 32 |
| Ca, mg kg-1 | 5,880 | 1,490 | 8,427 | 32 |
| Fe, mg kg-1 | 884 | 143 | 807 | 32 |
| Ash, mg kg-1 | 446,706 | 10,278 | 5.7 | 31 |
| NDF, mg kg-1 | 34,404 | 11,034 | 5.5 | 25 |
In study 2, we specifically chose hourly intervals for sampling to determine if the sampling time points of ~4 and 8 h used in study 1 had prevented us from best characterizing the performance of the MSPS from instantaneous samples at only two time points. Table 5 summarizes the amount of pH modifiers and MgCl used during runs in study 2. The volume of manure treated in study 2, and the amounts of sulfuric acid, MgCl, and NaOH were similar to that utilized during study 1. Oxalic acid was used as a sole source of acidification due to its ability to both reduce pH and its ability to bind Ca (Brown, et al., 2018). Bed production over the course of a run (~8 h) should represent the accumulative effect of any decreases and increases in bed formation, and can serve as a way to assess struvite formation vs changes in concentrations of ortho-P or total P in treated manure. In one 2-tank run when oxalic (125 kg) was followed by oxalic-sulfuric (125 kg and 19 L) we observed a gain in bed growth of 0.002 kg per 1000 L of manure processed (table 6). When oxalic was added at a more moderate rate of 33 kg and sulfuric at 42 L the bed growth increased to 0.38 kg/1000 L. Bed growth resulting from struvite crystallization can be masked by struvite losses from the bed due to dissolution during episodes of low pH, adherence of crystals to air bubbles passing through and out of the bed, and local up-currents exceeding the settling velocity of the smallest crystals due to turbulence. Each of these phenomena is typical during start-up of the system. These start-up losses become significant in comparison with growth by crystallization when operation amounts to only a few hours for each start-up.
| Table 5. Summary of amount of manure treated per run and use of pH modifiers – study 2. | ||||||
| Parameter | Average (L) | SE | SD | Min | Max | n |
| Treated volume of manure, L | 17,190 | 99 | 431 | 16,340 | 18,240 | 19 |
| Sulfuric acid, L | 35 | 2.7 | 11 | 19 | 53 | 18 |
| Oxalic acid, kg | 45 | 15.4 | 15 | 12.7 | 125 | 9 |
| MgCl, kg | 52 | 2.7 | 12 | 15.4 | 63.6 | 19 |
| Volume processed through cone, L | 12,230 | 167 | 729 | 10,260 | 13,110 | 19 |
| NaOH, kg | 41 | 3.7 | 9.1 | 28.5 | 54 | 7 |
| Aqueous ammonia, L | 66 | 3.4 | 12 | 42 | 87 | 13 |
| Table 6. Struvite bed production as affected by manure source and pH modifiers- study 2. | |||
| Run | Bed Growth (kg/1000 L) | Bed Type | Manure Type |
| Run 1- 2 tanks - oxalic-NaOH and oxalic-sulfuric-NaOH | 0.002 | Fine granular | Non-AD |
| Run 2- 2 tanks – oxalic/sulfuric-NaOH | 0.38 | Fine granular | Non-AD |
| Run 3- tank 1 - oxalic-NaOH | -0.99 | Fine granular | Non-AD |
| Run 4- tank 1 - sulfuric-NaOH | -0.50 | Fine granular | Non-AD |
| Run 5- tank 1 - sulfuric-NH3 | -0.39 | Fine granular | Non-AD |
| Run 6- tank 1 - sulfuriic- NH3 | 0.09 | Fine granular | Non-AD |
| Run 7- tank 1 - sulfuric- NH3 | -0.29 | Fine granular | Non-AD |
| Run 8- tank 1 - sulfuric- NH3 | -0.07 | Fine granular | Non-AD |
| Run 9- tank 1 -sulfuric- NH3 | 0.07 | Fine granular | Non-AD |
| Run 10- 2 tanks 1 –AD[a] -sulfuric-oxalic- NH3 | 0.17 | Small spherical | AD |
| Run 11- 2 tanks - AD-sulfuric-oxalic- NH3 | 0.68 | Small spherical | AD |
| Run 12- tank 1 - AD-sulfuric- NH3 | 0.70 | Small spherical | AD |
| Run 13- tank 1 - sulfuric- NH3 | 0.52 | Small spherical | Non-AD |
[a] AD = anaerobically digested. | |||
Figure 2 summarizes the response of % reduction in ortho-P in four runs that used oxalic acid, sulfuric acid, or a combination of the two acids. The October run using oxalic acid in a first run and then a combination of oxalic and sulfuric acid in a second run clearly shows that oxalic acid alone did not perform well in reducing ortho-P with negative reductions observed throughout the first 10 h. When sulfuric and oxalic were used in combination the reduction in ortho-P became positive and achieved an instantaneous reduction of 60% by the end of the run 2. The other runs shown in figure 2 also show good reductions in% ortho-P when either a combination of oxalic and sulfuric were used or when sulfuric was used alone, with maximal reductions of ortho-P of 60% or greater. A similar pattern was observed for the instantaneous reduction in % total-P for these same runs (fig. 3).
Evaluating the performance at hourly time intervals in study 2 allowed for the observation of the variability in performance of the MSPS, and hence whether data points of 4 and 8 h after initiation of a run as was done in study 1 provided the best estimates of system performance. Data in figures 2 and 3 clearly show that in the early hours after initiation of a run that an increase in concentration of ortho-P and total-P occurred. It was presumed that this was a result of some of the starter bed of struvite being dissolved and resulting in an increase in concentration of ortho-P and total-P in the manure stream, or small crystals of struvite not remaining in the fluidized bed and floating out as suspended particles. One possible reason for this observation was particle size of the initial struvite bed. To evaluate this assumption, we compared two forms of struvite bed. Summarized in figure 4 is the concentration of total-P in the manure stream when 2 forms of struvite bed were evaluated, a fine granular form or a small spherical. The data clearly shows that after initiation of a run with the fine granular form that there is an increase in total-P in the manure stream at h 2 and 4. This observation was not evident when the small spherical form was used as a source of bed.
 |
| Figure 2. The instantaneous percent reduction of ortho-P in hourly samples obtained during a time series run using oxalic, NaOH, and sulfuric acid as pH modifiers during October, November, and 11 December runs. Sulfuric acid and caustic soda only were used in the 17 December run - Study 2. October – raw manure/ tank 1 - oxalic acid addition, 125 kg oxalic, 11.25 h/ tank 2 - oxalic + sulfuric, 125 kg oxalic + 19 L sulfuric, 13 h, 19 L min-1 run rate. November – raw manure/tank 1 – oxalic acid, 33 kg - sulfuric acid, 38 l – 12.5 h/tank 2- oxalic acid, 33 kg - sulfuric acid, 45.6 L – 12.5 h, 16.7 L min-1 run rate. 11 December – raw manure/ tanks 1 – oxalic acid, 39.6 kg - sulfuric acid, 32.3 L – 13.25 h, 16.7 L min-1 run rate. 17 December – raw manure/tank 1 – sulfuric acid, 32.3 L – 13 h, 16.7 L min-1 run rate. |
 |
| Figure 3. The instantaneous percent reduction of total P in hourly samples obtained during a time series run using oxalic, NaOH and sulfuric acids as pH modifiers for October, November, and 11 December runs. Sulfuric and NaOH only were used in the 17 December run - Study 2. October – raw manure/ tank 1 - oxalic acid addition, 125 kg oxalic, 11.25 h/ tank 2 - oxalic + sulfuric, 125 kg oxalic + 19 L sulfuric, 13 h, 19 L min-1 run rate. November – raw manure/tank 1 – oxalic acid, 33 kg - sulfuric acid, 38 L – 12.5 h/tank 2- oxalic acid, 33 kg - sulfuric acid, 45.6 L – 12.5 h, 16.7 L min-1 run rate. 11 December – raw manure/ tanks 1 – oxalic acid, 39.6 kg - sulfuric acid, 32.3 L – 13.25 h, 16.7 L L min-1 run rate. 17 December – raw manure/tank 1 – sulfuric acid, 32.3 L – 13 h, 16.7 L min-1 run rate. |
Use of aqueous ammonia was evaluated as pH modifier to increase pH in the cone as it could be considered an organic source if derived from the processing of manure to capture ammonia. The average amount of aqueous ammonia used was 66 L or 5.4 L per 1000 L of manure (table 5). Figure 5 summarizes an example of the reduction in ortho-P and total-P when aqueous ammonia was used to raise the pH in the cone. After an initial increase in concentration at ~1 to 2 h of the initiation of the run (associated with the use of fine granular bed), a steady decline in ortho-P or total-P was observed with a 56% reduction in ortho-P after 6 h of processing. An additional example of the response when both aqueous ammonia and the spherical form of bed were used is shown in figure 6. This run was conducted with AD manure and the response in decreased ortho-P or total P was rather immediate and achieved a concentration of 75% of pre-treated concentration within the first hour of operation. These multiple observations with the spherical form of the bed seem to confirm that it is a preferable form as a starter bed in the cone.
 |
| Figure 4. Effect of bed source on total P and ortho-P concentration of undigested manure after passing through the fluidized bed - study 2. Data (May 2019) is from using the 0.9-mm spherical form of struvite. Data (February 2019) is from using the granular sand form of struvite. |
 |
| Figure 5. Summary of instantaneous concentration or ortho-P and total –P obtained during a time series run (2/15) using aqueous ammonia as a pH modifier and a fine granular form for the bed. The decrease in OP is demonstrated at hour 6 with a 56% reduction in OP - study 2. Raw manure – sulfuric acid, 53.2 L, - aqueous ammonia, 72.2 L, - 12.25 h, - 16.7 L min-1 run rate. |
 |
| Figure 6. Summary of instantaneous concentration or ortho-P and total –P obtained during a time series run (4/1) using aqueous ammonia as a pH modifier and a small spherical form for the bed. The decrease in OP is demonstrated at hour 1 with an ~ 75% reduction in ortho-P or total-P - study 2. |
The maximal reduction in ortho-P or total-P based on source of manure, AD, or non-AD is summarized in figures 6 and 7. The average reduction in ortho-P or total-P based on maximal reduction for non-AD manure (fig. 7) was 66% and 51%, respectively. In contrast the average reduction in ortho-P or total-P based on maximal reduction for AD manure (fig. 8) was 88% and 79%, respectively. Since it is known that AD of manure converts organic-P to inorganic-P (Ma et al., 2017) it would be expected that P extraction from AD manure would be 22% to 28% units greater than non-AD manure.
Comparisons to Alternative Treatment Systems
Physical Pre-Treatment
All manure treatment systems benefit from some level of solid-liquid separation prior to chemical treatment. Physical pre-treatment facilitates chemical mixing, lowers the buffering capacity of the manure to allow for pH adjustments needed, and allows heavier solid precipitates formed by the chemical treatment process to readily separate from the liquid phase and fall to the bottom. As described, the struvite system discussed accomplished this by using active and/or passive solid-liquid separation that existed on the farm, limiting testing to liquid manures containing less than 2% solids.
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| Figure 7. Maximal single point reduction in ortho-P or total P in non-AD manure as affected by pH modifiers – study 2. |
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| Figure 8. Maximal single point reduction in ortho-P or total P in AD manure as affected by pH modifiers – study 2. |
The USDA/Pennsylvania State University designed MAnure PHosporus EXtraction (MAPHEX) System accomplishes this physical pre-treatment on raw dairy manures by incorporating two stages of solid-liquid separation (a screw or auger press followed by a decanter centrifuge) into the mobile system (Church et al., 2016, 2017, 2018a, 2020). These two stages respectively remove about 80% and 10% of total solids (all stackable, 70% to 75% moisture), while simultaneously removing approximately 15% and 45% of total phosphorus respectively, from raw dairy manure. Given that typical raw dairy manures tested thus far range from 8% to 12% solids, the resulting liquid that is chemically treated is around 1% total solids (Church et al., 2016, 2017, 2018a). In cases where swine manure (which is typically lower in total solids) has been tested, it has been shown that the screw/auger press stage was unneeded for raw manures, and even the centrifuge was unnecessary if only manure liquors from a settling lagoon were treated (Church et al., 2020).
The newberyite system likewise uses passive physical pre-treatment, processing only manure liquors that are around 1% to 2% total solids (Vanotti et al., 2018).
Chemical Treatment and Total Phosphorus Removal
MAPHEX System
As opposed to struvite precipitation, the MAPHEX System relies on a simple sorption mechanism to convert ortho-P into a particle, and then rather than allowing the heavier particles to settle to the bottom and be recovered, the System utilizes an AutoVac® (ALAR Engineering Corporation, Mokena, Ill.) filtration unit using diatomaceous earth (DE) as a filtrate material to perform a final solid-liquid separation step (Church et al., 2016, 2017, 2018a, 2018b, 2020). Typically, the chemical treatment used has been ferric sulfate, but other chemicals (goethite, aluminum chlorohydrate, aluminum sulfate, calcium hydroxide, ferric chloride) as well as a byproduct of acid mine drainage cleanup, that is primarily goethite, have also been demonstrated to be equally effective (Church et al., 2016, 2017). In cases of iron and aluminum salts, the primary chemical reactions are presumed to be the formation of aluminum or iron hydroxides by reaction with water, followed by sorption of ortho-P to the surfaces formed. In the cases of goethite, calcium hydroxide, and the acid mine drainage byproduct, the presumed reaction is simple sorption alone. In all cases, no pH changes have been noted and the chemical mechanism appears not to be affected by the pH of the manures (Church et al., 2016, 2017, 2018a).
The MAPHEX System has been tested in two different configurations on both dairy and swine manures. Phosphorus removal by the full system has shown to remove greater than 95% of total P from dairy manures and concentrate it into compact, stackable solid forms that are 70% to 75% moisture (Church et al., 2016, 2017, 2018a, 2020). An adaptation of the full system that is capable of triple the flow-through rate of the standard full system through the use of a disc-stack centrifuge following the decanter centrifuge was shown to remove 75% of Total P from dairy manure at a lower cost per L (Church et al., 2016, 2017, 2018a). The reject from the disc-stack centrifuge (about 10% by volume), which is still a slurry, is then directed to the AutoVac® unit to yield a stackable solid (70% to 75% moisture). On swine manures, as noted above, the initial solid-liquid separation stages were shown to be not needed, and P removal efficiencies were greater than 96% (Church et al., 2020). In all cases, greater than 90% of N was retained in the liquid phase for beneficial use by the farmer (Church et al., 2016, 2017, 2018a, 2020).
Recovery of Ammonia and Newberyite-P with Gas-Permeable Membranes
In this process (Vanotti et al., 2018), phosphorus recovery is combined with ammonia recovery using gas-permeable membranes. In a first step, the ammonia and carbonate alkalinity are removed from digested livestock wastewater using low-rate aeration and a gas-permeable membrane manifold. In a second step, the phosphorus is removed using magnesium chloride (MgCl2). The N removal is done with low-rate aeration in the reactors that naturally increases the pH of the liquid and accelerates the rate of passage of NH3 through the submerged gas-permeable membrane manifold and further concentration in an acid stripping solution reservoir (Dube et al., 2016). After ammonia treatment, the effluent is low in ammonia, low in carbonate alkalinity, and has a higher pH. The removal of the soluble ammonia by the membrane produces acidity (H+) and destroys carbonate alkalinity at a rate of 4.1 mg L-1 of alkalinity for every 1 mg L-1 of ammonia removed (Daguerre-Martini et al., 2018). In turn, these conditions improve precipitation of phosphate minerals of high-grade. For example, Vanotti et al. (2017) reported phosphates produced from anaerobically digested swine manure using this process having high concentrations of P (20% P or 46% P2O5) and Mg (17%), and low concentrations of N (1.8%), Ca (0.4%), and K (1.7%). The resulting molar ratio of the P product was 1.0:1.1:0.2:0.0:0.1 for P:Mg:N:Ca:K, respectively, similar to the composition of the bio-mineral newberyite (MgHPO4.3H2O) found in guano deposits, which has approximately 18% P and 14% Mg and 1:1 P:Mg molar ratio.
Economics
Struvite System
A partial budget economic analysis was conducted based on the variable chemical cost to produce struvite. When using data from the runs with the highest recoveries of P, the cost as calculated on a per cow per d basis when considering chemical costs were: $0.22 (anaerobically digested manure) and $0.39 (undigested manure). When calculated on a per L basis of average manure produced per cow per day, the range was $0.003 to $0.006. The use of a cost per cow d-1 metric is a common approach in the dairy industry but it is not the most appropriate way to evaluate the cost to a given farm since the goal is to achieve a net-zero balance of P imports and exports from any given farm. The net-zero balance will be affected by factors such as: number of cows, number of acres utilized for manure application, phosphorus utilization by crops grown, double or triple cropping strategies, diet manipulation, and manure exported off-farm. The factor having the greatest impact on achieving P balance is the land base for growing crops that utilize manure. The first base scenario for comparison consisted of 67 g d-1 cow-1 excreted P, 202 ha of cropland, and 27 kg P uptake by crops per year and varying the number of lactating cows to achieve P balance on the farm with capture of P as struvite. The cost for capture of P as struvite from non-AD and AD manure was: 1000 cows - $698,922, $367,455; 750 cows - $304,549, $160,115; and 600 cows - $67,925, $35,711, respectively. The second base scenario for comparison consisted of 67 g d-1 cow-1 excreted P, 1000 cows, and 27 kg P uptake by crops per year and varying the number of hectares of cropland to achieve P balance on the farm with capture of P as struvite. The cost to capture P as struvite from non-AD manure was: 243 ha - $523,208; 283 ha - $347,494, 324 ha - $171,799; 344 ha - $83,922; and 362 ha - $4,851. The cost to capture P as struvite from AD manure was: 243 ha - $275,007; 283 ha - $182,693; 324 ha - $90,312; 344 ha - $44,122; and 362 ha - $2,550.
MAPHEX System
As described, the MAPHEX System is capable of removing up to 95% of P from dairy manure. The treatment cost for this level of treatment was originally 1.65 cents L-1 (Church et al., 2016). By employing an ashing procedure to recover the DE filtration media for re-use on the AutoVac® however, we found that daily operating costs could be reduced by more than 60% (0.66 cents L-1) (Church et al., 2018b). We further found that if the effluent from the decanter centrifuge (which operates at about 2500 times gravity) was directed to a disc-stack centrifuge (which operates at about 12,000 times gravity), the resulting effluent showed a 75% P reduction compared to the original raw dairy manure, a sufficient P reduction for most farms (Church et al., 2018b). Furthermore, directing the fluid reject from the disc stack centrifuge to the AutoVac® resulted in a stackable solid (Church et al., 2018b). While an AutoVac® is still used in this system, costs are greatly reduced to less than 0.26 L-1 treated since only 10% of the total volume of manure treated is directed to the AutoVac®. For swine manures, a pilot-scale study showed that costs would range from 0.24 to 0.95 L-1 without DE regeneration, and as low as 0.11 to 0.38 L-1) with DE regeneration (Church et al., 2020).
Newberyite + Ammonia Recovery System
The operational costs of ammonia recovery with the gas-permeable membrane system and subsequent phosphate recovery as magnesium phosphate (newberyite) were calculated on the basis of treating the anaerobically digested liquid swine manure from a typical 4000-head swine finishing farm in North Carolina growing pigs from 22.7 to 100 kg (50 to 220 lb) with a 245,000 kg (540,000 lb) steady-state live weight (SSLW). Treatment variables used in the calculations are based on the experimental work of Vanotti et al. (2017, 2018) obtained with anaerobically digested swine manure and these conditions: 1) raw swine manure is produced at a rate of 21.8 m3 d-1 (7,957 m3 yr-1) and contains 2,007 mg TKN L-1 and 494 mg TP L-1 (Vanotti et al., 2009); 2) after anaerobic digestion, 80% of the TKN and TP in raw swine manure is available as NH4+ and PO43- for recovery by the system; 3) N recovery efficiency is 98% and P recovery efficiency is 99%; 4) amount of H2SO4 needed to capture N calculated from mole ratio of ammonium sulfate (3.5 kg of acid per kg of N recovered), amount of nitrification inhibitor (niyrapyrin) used in the N recovery step is 22.5 mg L-1, and amount of MgCl2.6H2O applied as P precipitating compound is 3.0 g L-1 based in the P concentration and a Mg:P molar ratio of about 1:1.2 (Vanotti et al., 2018). Equipment needs for the N capture include membrane modules, feed pump, acid pump and controls, tanks, blower and piping as described by Dube et al. (2016) for a total annualized cost of equipment of $21,059. Equipment needs for the subsequent P recovery section include chemical feeding equipment, a 0.3 m3 chemical mixing chamber, an 8.8 m3 settling tank to separate the P precipitate, and dewatering unit to dry and bag the P product for a total annualized cost of P extraction equipment of $6,530 ($43,820 initial investment). The amount of NH4+ and PO43- available after anaerobic digestion for the 4,000-head swine operation is 35.1 kg N per day-1 and 8.6 kg P per day, respectively.
The operational costs to recover the N are calculated using an average recovery efficiency of 98%, with the amount of N recovered per year from this operation at 12,547 kg. The dosage of sulfuric acid (H2SO4) to absorb this N is 120 kg d-1 and the annual cost of acid is $14,053 (unit cost = $0.32 kg-1). For nitrification inhibitor, using nitrapyrin (commonly used for farming) at 22.5 mg L-1 concentration, the dosage is 0.5 kg d-1. The resulting cost of nitrification inhibitor is $1,794 per year (unit cost = $10 kg-1). Power consumption for the blower to provide low rate aeration (to increase the pH approximately 1 unit ~9.4 and facilitate membrane N uptake) is 13.7 kWh/d, and for influent and N module pumps is 26.4 kWh d-1 that amount to a total power consumption of 40.1 kWh d-1, resulting in an annual electrical cost of $1,020 (unit cost = $0.0698 kWh-1). Therefore, using the gas-permeable membrane method and low-rate aeration to increase the manure pH, the estimated operational annual cost (chemicals + power) for recovering the manure N in a 4,000-head swine farm that uses anaerobic digestion is $16,867. Expressed in terms of pig live weight in the farm (540,000 lb), the annual operational cost to recover the manure N is $31.23 per 1000 lb SSLW per year. In terms of the volume of manure treated (7,957 m3 yr-1), the operational cost to recover the N is $2.12 per m3 (0.2 cents per L).
The preceding removal of the N is needed to produce newberyite using the MgCl2 precipitating compound. Therefore, the operational costs to recover the P after N recovery are additional. They were calculated as follows: with an average P recovery efficiency of 99%, the amount of P recovered per year from this operation is 2,924 kg. The dosage of MgCl2.6H2O to precipitate the P as newberyite is 67.6 kg d-1 and the annual cost of the MgCl2 is $2,467 (unit cost = $0.10 kg-1). Power consumption in the P-module is that for the chemical dosing and chemical mixing that totals 7.20 kWh/d, resulting in an annual electrical cost of $210 (unit cost = $0.0698 kWh-1). Therefore, the additional operational annual cost (chemicals + power) for recovering the P (in addition to the N recovery costs) is $2,670, or $4.94 per 1000 lb SSLW per year, or $0.34 per m3 of liquid manure treated (0.03 cents per L).
The annual operational cost of the total treatment system that would separate and recover most of the manure N and P generated by a 4,000-head finishing swine farm is $19,534 per year. Expressed in terms of pig live weight in the farm (540,000 lb), the annual operational cost to recover the manure N and P is $36.17 per 1000 lb SSLW per year. In terms of the volume of manure treated (7,957 m3 yr-1), the operational cost to recover the N and P from the manure is $2.45 per m3 (0.2 cents per L of manure treated). Therefore, most (86%) of the total cost is in the first step to recover the N and once the N is removed, it is relatively inexpensive (0.3 additional cent per L) to recover the P because the chemistry of the wastewater is changed with the preceding N step, such as lower ammonia, lower carbonate, and higher pH of the wastewater, conditions that favor the precipitation of P using MgCl2 compounds. The amount of ammonium sulfate potentially recovered during one-year operation of the N recovery module (12,547 kg N) has an equivalent fertilizer value of $34,002 assuming a value of $2.71 per kg N as ammonium sulfate ($522/ton). The amount of P potentially recovered in the P precipitates (7,129 kg P2O5 per year) has an equivalent fertilizer value of $6,701 assuming a value of $0.94 per kg P2O5. Therefore, the combined value of N and P products recovered annually from the manure produced in the 4,000-head swine farm using the New-P process is $40,703.
Conclusions
The results of evaluation of struvite production on >30 dairies suggest that the overall best conditions for reduction of P from dairy manure occurred when Ca and solids were limited in the manure, AD manure was used, and with sulfuric acid or a combination of sulfuric and oxalic acids, and aqueous ammonia, as pH-modifiers. In addition, using an initial bed of small spherical form of struvite seemed to provide for better retention of struvite crystals in the cone for subsequent harvest. The economic performance was best with AD manure due to the greater concentration of inorganic P compared to non-AD manure.
Based on the system performance reported and assumptions, the three systems provide an initial end product for off-farm export in the range of $0.003 to $0.02 per L of manure. A complete economic analysis (when data is available) of the technologies would need to consider labor costs, operator training, lab analysis, as well as other benefits such as reduction of greenhouse gas emissions due to reduction of direct and indirect N2O emissions. Additionally, consideration would need to be given to the reduction of land area required on the farm to utilize treated manure effluents once the nutrients are separated, concentrated and moved economically to other farms that need the nutrients.
Acknowledgements
Financial support was provided through a USDA-NRCS conservation innovation grant 69-3A75-17-51 and the Washington State Dairy Industry. We would like to thank the dairy producers of Washington who collaborated with us on this project. Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the USDA. The technology for phosphorus recovery by the MAPHEX System is covered by U.S. Patent No. 10,737958 and Patent Application No. 62/727,732-DN. 77.16. The technology for recovery of ammonium and newberyite-P (New-P) is covered by U.S. Patent No. 9,926,213 B2.
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