Nazim Cicek Page ARDI Final Report. Dr. Nazim Cicek, Biosystems Engineering, University of Manitoba

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1 Nazim Cicek Page ARDI Final Report Project #: PHOSPHORUS RECOVERY THROUGH STRIUVITE PRECIPITATION FROM RAW AND ANAEROBICALLY DIGESTED HOG MANURE Dr. Nazim Cicek, Biosystems Engineering, University of Manitoba Objectives: The aim of this research was to find the optimum physical parameters and process technology for precipitation of struvite from liquid hog manure. A simple and economic process would facilitate better N:P ratio for land-applied manure as well as potentially create an income stream for Manitoba farmers. This report gives a synopsis of the research findings and experimental details in subsequent sections. Peer reviewed journal articles have been published (Section 1) or are currently in preparation for dissemination. Results have also been presented in international scientific conferences. Background: Application of hog manure to croplands can lead to soil build-up and eventual runoff of phosphorus to surface waters causing environmental damage. Reclamation of P from manure can potentially solve this problem as well as create a slow release fertilizer called struvite (MgNH 4 PO 4 ). All of the components are present in liquid manure but need to be in dissolved form to form the struvite crystal. Precipitation involves raising the ph, as struvite solubility decreases in basic conditions. A complicating factor is the presence of calcium in manure, which can interfere with struvite formation and produce other less useful compounds. Most struvite process technology uses centrate from municipal waste water treatment plants and would not have application with high solids liquid hog manure. Struvite formation readily occurs under conditions where soluble magnesium (Mg), ammonia (NH 4 ) and phosphate (PO 4 ) reach super saturation. Solubility decreases as ph increases, forming struvite crystals that can increase in size by crystal growth or by agglomeration and settle out of solution. Most pilot reactors use upflow fluidized bed configurations but these are capital and operationally intensive, increasing the cost of P removal and making the process inhibitory for most farm operations. In theory, a nutrient rich supernatant in a simple batch reactor needs only alkali ph adjustment and adequate settling time for a low energy, low capital, reactor system. Synopsis: With the help of ARDI funding, we have accomplished important research that establishes the direction of phosphorus reclamation using Manitoba liquid hog manure. These tests have used raw and anaerobically digested manures to determine optimum chemical additions and processes best serving onfarm application of this technology. Chemicals additions usually include an alkali to increase the ph to the point of struvite precipitation (ph 8-9) and magnesium addition when it is the limiting component. The use of these chemicals is benign but expensive, and is a major part of costing studies conducted on struvite production from manures. Evaluating phosphorus reclamation without the use (or limited use) of chemicals was important in our research. Tests of anaerobically digested and raw manures verified that good total P removal was possible (80%) with centrifuged supernatant. It was also established the addition of magnesium aided P removal and that while higher ph adjustment (ph 9) removed more P, the purest struvite was collected at lower ph levels (ph 7.5). The details of this study are described in section 1. Removal of P requires struvite 1

2 Nazim Cicek Page constituents to be in a dissolved form and the storage of raw manure can affect the concentration of these nutrients. Using natural fermentation to lower ph is very low cost compared with chemical addition. We determined a fermentation step of 8-12 days will increase the acidity of the manure and thus increase available nutrients for struvite production. In the batch tests we conducted, struvite making potential increased from 2.3 to 2.8 fold (depending on the use of stirring) when compared with raw manure. This highlights the need for careful manure management but also the efficiency gain in P removal that is possible with no added chemicals. This experiment is described fully in section 2. From the wide range of struvite precipitation processes available to test, tank mixing with gravity settling was selected as the most economical in terms of capital infrastructure and operating expertise. The high solids content of primary lagoon supernatant posed too many challenges to use an upflow reactor as it is used for precipitation of P from municipal centrate. The construction of a pilot-scale reactor to process lagoon supernatant and produce a struvite rich product was successful on several counts. We established 70% TP could be removed without addition of MgCl 2 and the sludge produced could be dried yielding a product with reasonable TN and TP content (both about 6%). Solution ph was raised with KOH instead of NaOH, thus keeping sodium out of the system and adding a useful nutrient at no significant cost increase. See section 3 for details. Additional work is needed to decrease organic solids in the final product and on economical means of drying the sludge. There is still work to be done in refining the results and in combining these methods to make an optimized manure management system. These results show that large infrastructure expenses may not be needed to remove P from liquid manure and that simple, appropriate technologies can replace expensive chemical additions. Future work will test air sparging as a means of ph increase and the use of flow through reactor systems that can be operated continuously. Section 1 Phosphorus removal from anaerobically digested swine wastewater through struvite precipitation The objective of this study was to determine the optimum conditions for struvite precipitation from anaerobically digested swine manure. Specific goals were to determine the effect of ph and Mg:P ratio on struvite precipitation, and to establish the rate constant for struvite precipitation from anaerobically digested swine manure. The recovered precipitate was examined to determine struvite quality and presence of co-precipitate. Methods Anaerobically digested swine manure effluent was collected from controlled experimental digesters, screened, diluted and then centrifuged. Samples of the centrate were analyzed to determine its basic constituents. Based on the initial Mg:PO 4 -P ratio of the centrate, appropriate amounts of MgCl 2 were added to four beakers containing 2 l centrate each to create Mg:P ratios of 1.0:1, 1.2:1, 1.4:1, and 1.6:1. The four batches of adjusted effluent were divided between five glass beakers each so that there was 400 ml in every beaker. To adjust the centrate ph in a range from 7.5 to 9.5, either 1.0 M NaOH or 1.0 M 2

3 Nazim Cicek Page HCl was added with a dropper as appropriate to each beaker, while monitoring the ph with a probe. Consequently there were four sets of five beakers with a similar ph range and different Mg:P ratios. After the equilibration period the final ph of each beaker was measured with a probe, then 5 ml samples were taken for ICP and FIA analyses. Due to the high ammonia concentrations, samples for FIA were further diluted ten-fold before analysis. The dried precipitate on the qualitative filter paper was weighed and homogenized. Samples of these crystals were analyzed using X-ray diffraction (XRD) Results The experimental objectives were achieved by studying the effect of two factors, Mg:P ratio and ph. Removing more phosphorus at higher Mg:P ratios would allow a given TP concentration to be achieved at lower ph values in the centrate, thereby minimizing the amount of ph adjustment needed. Other authors reported optimum Mg:P ratios between 1.3:1 and 1.6:1, similar to results from this experiment where the lowest phosphorus concentration was found in the Mg:P ratio of 1.6:1. It has to be kept in mind, however, that there will be a cost associated with increasing the Mg:P ratio, and so a balance has to be found between magnesium amendment and ph adjustment that will minimize cost and maximize removal. As shown in Fig. 1, the minimum TP concentrations were found to occur at ph 9.0 for all the Mg:P ratios. The increase in TP at ph 9.5 could be due to the solubility of struvite changing with ph, since the speciation of phosphate and ammonium is ph dependent. The solubility of struvite generally decreases with increasing ph, but starts to increase at higher ph values because the ammonium ion concentration decreases while the phosphate ion concentration increases. Therefore, struvite will have a ph of minimum solubility, where the greatest amount of precipitation will occur. The greatest TP removal achieved was 80%, at ph 9.0 and a Mg:P ratio of 1.6:1, for a final phosphorus concentration of 7.9 ppm. The TP concentrations at the three highest ph values were very similar, suggesting comparable precipitation. This can be further illustrated by looking at the mass of recovered crystals for the various ph values, as shown in Table 3. There is a dramatic increase in precipitate mass from ph 7.5 to 8.0 and from ph 8.0 to 8.5, almost doubling each time. However, the mass of precipitate recovered in ph 8.5, 9.0, and 9.5 does not differ as dramatically as the first two, they are in fact very similar. These findings support the results presented in Fig. 1, which shows that significant precipitation starts to occur around ph 8.5 and continues through to ph 9.5, with peak precipitation occurring close to ph 9. This agrees with the findings of Nelson et al. (2003), who found that maximum PO 4 -P removal occurred between ph 8.9 and Another factor that should be taken into account when looking for the optimum ph is the purity of the recovered crystal. Studies have indicated that high calcium concentrations in the centrate could lead to the formation of calcium phosphates or calcium carbonate. If calcium phosphates were formed, the amount of struvite recovered would decrease, along with the precipitant s value as fertilizer. 3

4 Nazim Cicek Page Figure 1. Comparison of TP concentration to centrate ph after 24 hours equilibration for different Mg:P ratios. If calcium carbonate was formed, it could interfere with struvite formation, but would not compete for phosphorus with struvite. It was found that the concentrations of struvite constituents all decreased with increasing ph, however, it was also evident that the calcium concentration dramatically decreased with increasing ph, suggesting that some form of calcium did precipitate. In order to determine if struvite was indeed recovered, and to assess the purity of any recovered struvite, XRD was used to analyze the recovered precipitate. The XRD analysis shows that the purest struvite was recovered from the centrate with a ph of 7.5, since the intensity peaks of the analyzed substance correlate well with those of pure struvite, as shown beneath the graph. Although struvite is still prominent for the ph 8.0 precipitate, it also has peaks consistent with calcium carbonate, or calcite. A similar trend can be observed for the ph 8.5, 9.0, and 9.5 precipitates. There was no evidence of an amorphous phase in any of the spectra, which would have suggested the presence of calcium phosphates. This seems to indicate that the calcium in the centrate precipitated mainly as calcium carbonate. Conclusion The optimum conditions for maximum phosphorus removal from anaerobically digested swine manure were found to be ph 9.0 and a Mg:P ratio of 1.6, where 80% of TP was removed. Although struvite was recovered at all ph values, molar balances and XRD analysis showed that the purest struvite was recovered at ph 7.5, since at higher ph values the precipitate also contained calcium carbonate. Consequently the optimum ph for struvite precipitation will depend on an acceptable trade-off between maximum phosphorus removal and greatest struvite purity. Overall it can be argued that, based on the amount of phosphorus removed and the quality of the recovered precipitate, struvite precipitation could be a viable method for phosphorus removal from anaerobically digested swine manure. 4

5 Nazim Cicek Page Section 2 Anaerobic fermentation of swine manure to increase P removal by struvite precipitation Reclamation of phosphorus from hog manure as a precipitate requires it to be in the soluble form (P- PO4). Liquid hog manure typically holds from 10 to 20% of total phosphorus (TP) in this form, the rest being associated with the manure solids. The working hypothesis is that acidic conditions arising from fermentation of manure will dissolve P from the solid phase in a significant amount. The purpose of this experiment was to investigate the effect of anaerobic fermentation and resulting acidification on the abundance of P-PO 4 and other constituents important for struvite precipitation (Mg, Ca, NH4) in hog manure. Additionally, the effects of stirring manure continually was compared with minimal mixing of manure, which would be closer to collection pit in a hog barn. Methods Fresh manure was collected from the Glenlea Research farm from within the stalls. This was fairly dry manure without washwater and a minimum of urine. This was then mixed with tap water into a slurry (representative of liquid manure) and frozen in one litre containers making a uniform supply for the duration of the experiment. Anaerobic fermentation of manure depends on microbial activity to break down volatile solids, producing volatile fatty acids (VFAs) and carbon dioxide as acidic by-products. The extent of this acidification was tested in batch mode with two four litre reactors stirred at RPM and one four litre reactor that was not stirred. The reactors were filled with manure and kept anaerobic (measuring ORPs of mV) and ph measurements were taken daily. When maximum acidification was reached, the reactors were fed manure to simulate a flow-through tank receiving manure and discharging effluent. Three separate hydraulic retention times (HRT) were tested: four day, eight day and 12 day and two cycles of each HRT elapsed before the effluent was sampled. Sampling consisted of triplicate samples of TS, VFAs, alkalinity, and nutrients of interest (P, Mg, NH 4, Ca) from filtered manure (0.45 µm), unfiltered manure in a weak acid solution, and digested manure. A mass balance of P in the manure forms was conducted on influent and effluent as a side experiment. These forms were liquid, solids that settle in one minute (three minutes for effluent) and floc that settles in 15 minutes. Volume of each of these settled fractions was measured and the fraction was sampled for TP analysis by digestion. Results The stirred reactors behaved in a similar way during the experiment. The unstirred reactor was only sampled for the liquid portion due to difficulty in getting a good mixed sample with the peristaltic pump. ph: The batch mode produced lowest reactor ph (Fig.1) after 12 days in Stirred 1 (ph 5.61) and 15 days in Stirred 2 (ph 5.75) and after 19 days in Unstirred (ph 5.84). After two cycles of 4d HRT both stirred reactors had a ph of The 8d HRT produced a ph of 5.84 and 6.08 in Stirred reactors 1 and 2 respectively. The 12 d HRT produced ph of 5.84 and 6.01 in the stirred reactors. Values recorded in the unstirred reactor were at times variable because of inconsistent effluent solids removal before the new manure was added (Fig. 2). 5

6 Nazim Cicek Page ph of S+rred Manure Fermenters ph S-rred 1 S-rred Days Figure 1. Stirred reactor response to anaerobic fermentation in batch mode (no addition of new manure), and in reactors fed new manure at the rates of 4, 8 and 12 day total volume exchange (HRT). Phosphorus forms: Using P data from the filtered, unfiltered and digested analysis P forms were determined. Dissolved P increased relative to TP in all reactors with increasing HRT. This was apparent with mixed manure as well as the liquid fraction of stirred reactors as well as the unstirred. TP of the liquid fraction increases from about 200 mg/l in the influent to about 400 mg/l in the effluent of each HRT. Unstirred manure liquid remains at about 200 mg/l. The increase of P-PO 4 translates into increased struvite making potential. When data from all struvite components (0.45 µm filter) was converted to molar mass, struvite potential increased from 2.6 mm to 7.4 mm with 12 d HRT (Table 1). Because there was little urine in the manure, sometimes NH 4 is the limiting nutrient. The molar ratio for struvite is 1:1:1 PO 4 :Mg:NH 4. Calcium is of interest because it inhibits struvite formation when the Ca:Mg ratio exceeds 0. Discussion/Conclusion: The experiment succeeded in proving anaerobic fermentation is effective in decreasing the ph of manure and that this change causes a significant increase in P-PO 4. The amount of struvite that could be precipitated from fermented manure increased from 10% of TP in influent to 25% in the 12 d HRT effluent, assuming all P-PO 4 could be successfully removed as struvite. Increased HRT did result in higher P-PO 4 levels but less so with increasing time. The difference in stirred and unstirred manure was mostly seen in the levels of TP in the liquid portion, being lower in the unstirred manure because low TSS. Consequently, the proportion of dissolved P was very high, making it good material to put through a fluidized bed reactor. 6

7 Nazim Cicek Page Table 1. Liquid fraction soluble nutrients from raw influent manure and effluent from each HRT. Theoretical struvite recovery is calculated from the limiting nutrient from each HRT. Nutrients in Filtered Manure (mm) mm of struvite possible influent 4 day 8 day 12 day influent 4 day 8 day 12 day Stirred 1 TP ICP Mg Ca NH Stirred 2 TP ICP Mg Ca NH Unstirred TP ICP Mg Ca NH Section 3 PHOSPHORUS REMOVAL AND RECOVERY FROM HIGH CALCIUM HOG LAGOON SUPERNATANT USING A GRAVITY-SETTLED STRUVITE REACTOR Struvite reactors using hog manure supernatant report varying degrees of P removal efficiency and unfortunately, incomplete data collection restricts full comparisons between studies. The most efficient reactors use supernatant low in TS and a high proportion of TP as P-PO 4. It may be noted that high P-PO 4 removal is not always reflected in TP removal. This may be from loss of P-PO 4 from calcium phosphate formation but subsequent loss of the particles to effluent. Implementing economical P removal at farms requires use of existing infrastructure without additional processes. High strength supernatants with high TSS and dissolved solids may keep struvite crystals and calcium phosphate in suspension even after they have precipitated. For phosphorus reclamation to be adopted on a wide scale the process of precipitation must be simple and inexpensive, so that common farmers can operate the equipment with a minimum of capital expenditure. This study takes a low capital, low energy input approach to precipitation of phosphorus from lagoon supernatant and uses a gravity settling system that removes the majority of TP and produces a nutrient rich sludge that can be processed into a dry fertilizer. The focus was on the function of the system and the precipitated product in terms of nutrient content and struvite purity. 7

8 Nazim Cicek Page Methods A commercial feeder barn in southern Manitoba using a primary and secondary lagoon system was selected to install the struvite precipitation reactor system. Two 410 litre cone bottomed plastic tanks equipped with two-way drain valves were installed adjacent to a full scale primary lagoon. Tanks were filled from the top with a hand operated diaphragm pump, dosed with KOH (45%) to reach ph 8.5 and allowed to settle for 24 hrs. Settled precipitate sludge was drained into separate containers and remainder of the effluent was released to the secondary storage (Fig. 1). It was found that an additional 24 hr. settling would further stratify the thicker grey sludge layer. The collected sludge was stored in the lab at 4 C and later dried in the sun in flat shallow trays and then pulverized in a ball bearing mill to produce the final product. Samples of influent and effluent were measured for TS, TSS, alkalinity as well as TP, P-PO 4, Mg, Ca and NH 4. Fig. 1. Schematic drawing of the gravity settling batch reactor. The tank is filled with lagoon supernatant, dosed with KOH to ph 8.5 and allowed to settle for 24 hrs. A two way valve allowed precipitate to be collected (~20 L/batch) and then effluent to be released to the secondary lagoon. Collected precipitate was dried and milled to a powder. Results Lagoon nutrient levels fluctuated considerably over time. Fall and spring sampling revealed significant changes, as did sampling from mid and late summer (Fig. 2). A 4 week period elapsed between two reactor operation events in July and August During this time, concentrations of P, Mg and Ca in unfiltered reactor influent increased by 281, 278 and 299 percent. Reactor performance was essentially unchanged however, removing 79 vs 82 % of P, 68 vs 67% Mg and 31 vs 35% Ca in unfiltered samples, respectively. Replicate samples were from consecutive batches of influent (n=3 in July and n=4 in August) and effluent samples (n=6 July and n=4 in August). 8

9 Nazim Cicek Page mg/l July Influent July Effluent Aug Influent Aug Effluent 0 P Mg Ca Fig. 2 Reactor performance with supernatant of different nutrient concentrations. Over a 4 week period nutrients in unfiltered influent samples increased by 280, 278 and 300% for P, Mg and Ca. Dosing with KOH to ph 8.5 reduced effluent concentrations of the nutrients by similar percentages. A possible cause of the summer nutrient increase was a floating straw cover placed over the lagoon by the farmer to reduce odours. Small daily changes in nutrient content appeared to correspond with the extent of straw cover at the reactor intake pump. Straw moved in the lagoon according to wind direction. Reactor operation removed the majority of supernatant P at the lower and higher P levels, reducing TP by 70% in digested samples (Table 1). Table 1. Nutrient removal in the struvite reactor as determined by total digestion of influent and effluent (n=3). TP (mg/l) Mg (mg/l) Ca (mg/l) Influent 216 ±4 142 ±3 467±21 Effluent 63.6 ± ± ±117 % reduction Removed nutrients (mm) The use of three different sampling methods (total digestion, unfiltered and filtered) enabled differentiation of the P forms present in supernatant of a limited number of batch samples. Precipitation reduced all forms to some extent but the greatest reduction was found in the dissolved P and mineral particulate forms (Table 2). Table 2. Phosphorus forms present in reactor influent and effluent. Influent (mg/l) Effluent (mg/l) Dissolved reactive P 95 8 Dissolved organic P 9 3 Particulate mineral Particulate organic Total P

10 Nazim Cicek Page Total suspended solids were reduced by reactor operation. Typical influent TSS was 4.9 ± 0.4 g/l and effluent was 2.17 ± 0.53 (N=3). Inorganic solids remained about the same (0.6 g/l) in both influent and effluent, indicating the loss to the precipitate fraction was mostly organic solids. Analysis of the air dried precipitate showed mass loss was 65.9% ±0.14 (N=6) by ignition at 550 C. There is difficulty in assessing organic carbon loss because mass is also lost from hydrates and other volatiles such as ammonia. Elemental analysis of dried precipitate showed concentrations of K, P and N to range from 3.8 to 6.3 % (Table 3). Determining the content of pure struvite formed in the reactor is difficult and can only be approximated by measuring the content of struvite constituents. To accomplish this, precipitate was mixed with distilled water and the ph adjusted with HNO 3 and allowed to equilibrate at ph 1.5 for 24 hrs. This allowed mineral particulates to dissolve into solution. When comparing results to nutrients measured by total digestion, all Mg dissolved at ph 1.5, suggesting no Mg was associated with organic particulate forms, as in the case of P and NH 4. For these reasons Mg was taken as the indicator of struvite and total struvite content was calculated according the Mg concentration. Other mineral Mg compounds may be present in the precipitate, but were not found using XRD analysis indicating if present they were in a non crystalline state. Table 3. Elemental analysis of air dried reactor precipitate. Values are given as a percentage of total mass and the percentage soluble at ph 1.5. Maximum struvite content is estimated as the mole fraction of the least abundant quantity soluble. Struvite molar wt is 244 g/mole. P Mg Ca TN K Na % total 5.6 ± ± ±0.34 % 6.3 ± ± ± 0.02 digestion % soluble at 4.5± ± ± ± ± ± 0.01 ph 1.5 Moles in g Possible struvite in 100g (g) A variance was found in nutrient concentration of lagoon supernatant over time but the proportion of P removed from precipitation remained similar. The variance may be from changes in diet, age of the pigs, or the use of a straw cover on the lagoon during the latter operation time. Precipitation within the reactor removed most of the P-PO 4 in the supernatant (92%) and additionally, 68% of the mineral particulate P settled out of suspension. It is possible that mineral particles settled by flocculation along with newly formed precipitate or by organic solid settling. The common assumption is that only P-PO 4 can be removed from supernatant because it can precipitate, but this data indicates other processes play an important role in P removal. Conclusion The gravity settling batch reactor removed 70% of TP with a 24 hr settling period. P was removed by precipitate formation as well as other settling processes. The settled sludge was difficult to dry and unpleasant to work with, due to the high ( ~50%) organic solids content. The struvite purity was estimated to be 33% assuming all of the Mg content was attributable to struvite crystals. As far as total nutrient content is concerned, the precipitate contains 5.6% P, 6.3% N and 3.8% K. Pure struvite would have

11 Nazim Cicek Page % P, 5.7% N and 0% K. High Ca concentration in the supernatant did not impair the N content of the product and the use of KOH as an alkali contributed favourably to the value as a fertilizer. 11