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PHOSPHORUS RECOVERY FROM LIVESTOCK EFFLUENTS
In Europe, P flow from animal manure represents 1.5 times the amount of chemical P fertilizer applied (1.6 and 1.1 MT-P/year) (Withers et al., 2015). In France, both flows are approximately equal (0.31 and 0.29 MT-P/year) ((Senthilkumar et al., 2012). The P balance in soils of Europe and France is still largely in excess despite a significant decrease in the last 30 years. Indeed, 42% of P currently applied is not exported and accumulated every year (21% in France). As described previously, these mean values hide the massive spatial variations in P applied and soil P accumulation/deficit across Europe and between regions of a same country (Csatho and Radimszky, 2009; GisSol, 2011), mostly due to livestock density and associated manure application in the fields nearby. Therefore, the challenge is to recover manure P from intensive livestock regions under a form that can be transported easily and at low cost to regions with soil P deficit. Intensive pig breeding is the primary livestock production in need of manure processing (Schoumans et al., 2015) due to its enclosed nature, without enough arable land for spreading. Currently, excess pig manure is exported mostly as compost for the solid phase, while the liquid phase undergoes nitrification/denitrification for nitrogen removal. Very few technologies for P recovery from pig manure or any livestock effluent currently exist on full scale. Therefore, this section will describe both these full scale processes and the one only tested at lab scale. The types of P recovery techniques for animal manure are similar to the ones in municipal WWTP. Thus P can be recovered from the liquid phase of digested or undigested manure, P can be recovered after pre-treatment of manure to extract P from, or concentrate in, the solid phase, and finally P can be converted into ashes through incineration.
Phosphorus recovery from the liquid phase
Two full scale units currently exist to recover phosphorus from animal manure. The first one treats 115,000 m3 of denitrified calf manure in Putten (Netherlands) since 1998, producing K-struvite. The process consists in 3 consecutive CSTRs, a clarifier and a struvite buffer tank (Schuiling and Andrade, 28 1999). pH is increased to 9 using MgO. Few details are known regarding the influent or effluent characteristics. Since the manure has already been denitrified, ammonium concentration is very low. As a result, the rise in pH leads to the precipitation of K-struvite, a process that would not occur at high ammonia concentration due to the lower solubility product of N-struvite compared to K-struvite. No information is given concerning the proportion of dissolved P vs. TP or the recovery rate. The solid recovered had low calcium or ammonium contamination (<2%) and a relatively low amount of organic matter (5.5%). Suspended solid concentration had a negative impact on the process when above 1 g/L, a phenomenon also noticed by Shen et al. (2011) and Zeng and Li (2006). Cattle slurry has a very high suspended solid content, and even after centrifugation the supernatant can still contain 10 g-TSS/L or more (Zeng and Li, 2006). The negative impact of suspended solids on struvite precipitation has also been observed in the case of pig slurry, with a decrease in the kinetics of precipitation, contamination of the solid obtained and smaller crystals (Capdevielle et al., 2016; Cerrillo et al., 2015; Zhang et al., 2012) The second full scale unit is operating in North Carolina, recovering phosphorus as calcium phosphate from the supernatant of centrifuged raw swine manure since 2003 (Vanotti et al., 2007) (Figure 12). The liquid phase undergoes nitrification denitrification in order to remove nitrogen as N2 gas and reduce alkalinity. Then, lime (Ca(OH)2) is added to adjust the pH to 10.5-11 and promote calcium phosphate precipitation. The solid obtained is withdrawn from the settling tank afterward and filtered with the help of an anionic polymer. N removal and associated alkalinity consumption helped reduce the amount of lime needed to 567mg/L, for 118 mg-P/L were removed. The crystallization step enabled a removal rate of 95% of soluble P entering the reactor. However, most of the phosphorus has been removed during the initial solid liquid separation step. As a result, only 20% of TP contained in the raw slurry is recovered as calcium phosphate. P contained in the solid phase is composted at a centralized solid processing plant, and therefore can be applied on nearby arable soil. However, compost is not as concentrated in P as a mineral fertilizer and cannot be transported very far away. The solid obtained contained 10.7%-P and a moisture content of 77%. No details were given regarding the use or commercialization of the product.
Hydrothermal treatment and acid leaching of P from manure
These pretreatment can be conducted on the raw manure itself and it does not require concentration/drying processes. Ekpo et al. (2016) studied the effect of temperature and type of acid or base on P dissolution in batch experiments lasting one hour, at 120, 170, 200 and 250°C. Without acid, temperature had a negative on P dissolution, with 500 and 10 mg-P/L after the 120°C and 250°C treatment respectively. The best results were obtained at 170°C with 0.1M H2SO4.79% of TP initially present in the manure was dissolved, 92% of it as ortho-P. The ratio of sulfuric acid to dissolved P was 5.6 Kg-H2SO4/Kg-P, a value similar to what is obtained by direct acidification on raw manure. It
35 should be noted that in this experiment, the swine manure was dried at 60% and re-dissolved afterward to undergo thermal treatment. Consequently, it was not direct use of raw manure. In a similar experiment, Heilmann et al. (2014) tested hydrothermal treatment on pig slurry during 40 minutes runs at 200-260°C. Hydrochloric acid was not added before treatment, but used instead on the char obtained. 90% of the P contained in the char was extracted jointly with calcium, indicating that most of the calcium in the pig manure was initially present as calcium phosphate. The acid to P ratio to reach this level of extraction was 108 Kg-HCl / Kg-P. Struvite was tentatively precipitated from the acid extract by adding stoichiometric amounts of Mg and NH4 +, but only calcium and phosphate precipitated. It appears that extraction of P from biochar is too expensive and the product obtained does not possess sufficient added value. Hydrothermal treatment of cow manure was evaluated in two consecutive studies by Dai et al. (2015,2017) with the objective to immobilize P in the char in the first experiment, and extract it with hydrochloric acid in the second experiment. Hydrothermal treatment alone (200°C, 4-24 h) enabled a large increase in phosphorus content in the solid phase and resulted in a lower extractability. This resulted from a conversion of water-extractable P and Ca into apatite. Depending on the objective for manure processing, hydrothermal treatment could be used as a mean to concentrate and immobilize phosphorus into the char. In the second study, Dai obtained the full extraction of TP with a 2% HCl solution. More than 90% of dissolved P was present as ortho-P. The acid to P ratio was strikingly low, with 2.2 Kg-HCl per Kg-P. There might be some ambiguity regarding the volume in which dissolved P is measured. No explicit value for TP in the initial manure was given. If the acid to P ratio is exact, the manure would have a P content of 4.2% P (% DM), a surprisingly high value, since most cow manure have a P content around 1-2% of dry matter.
Microwave heating (MW), acidification and wet oxidation of dairy manure with hydrogene peroxide
Pan et al. (2006) applied H2SO4 + MW + H2O2 pre-treatment to dairy manure based on its success on domestic sludge. The results indicated that MW in itself does not affect P dissolution, contrary to what occurs with EBPR sludge. Cell lysis induced by MW is efficient to release P from PAO in EBPR sludge but has no effect on inorganic P as calcium phosphate in dairy manure. Only temperature at 170°C, H2SO4, and H2O2 treatment led to the release of phosphates. MW only seemed to slightly accelerate the effect of H2O2 on P dissolution. No details were given regarding the exact amount of sulfuric acid used, making it impossible to calculate the ratio of acid to dissolve phosphorus. Qureshi et al. (2008) similarly found a positive effect of temperature but no effect of hydrogen peroxide on P dissolution during MW treatment in combination with H2SO4. Unfortunately, the effect of H2SO4 alone is never presented in any of the publications on MW treatment of cow manure. Again, the exact concentration of H2SO4 was not given. However, pH was decreased to 1.4, indicating a rather large amount of acid and making acidification the main reason for P dissolution. Phosphate concentration increased from 40 mg/L (or 20% TP) to 160 or 86% at 170°C, with and without H2O2.
36 Less efficient results were obtained by Jin et al. (2009). MW treatment alone did not lead to any dissolution of P. With 2% H2SO4 + MW, dissolved P increased from 20%-TP to 39%. A similar result was obtained with a lower amount of H2SO4 (0.5%) and the addition of H2O2 (1%), indicating that both acidification and wet oxidation increase P dissolution. It should be noted that if the H2SO4 added at 2% v/v is pure acid, then the acid to dissolved P ratios are extremely high, with 80-320 Kg-H2SO4 per Kg of P. The pre-treatment was used for both struvite precipitation and anaerobic digestion. MW alone did not increase the methane production, and H2SO4 logically inhibited anaerobic digestion due to sulfur inhibition. Cow manure is extremely challenging regarding solid liquid separation (Hotaling, 2006), as described earlier. The resulting high suspended solid concentration severely impedes struvite crystallization. In order to obtain a liquid with low SS content and high ortho-P/TP ratio, Zhang et al. (2015) tested several pre-treatments on liquid cow manure. An unknown amount of sulfuric acid was added initially to dissolve P by lowering the pH to 3-4. The acidified manure underwent a microwave (MW) process with the addition of H2O2. Solid and liquid were separated through a simple clarification stage.
Microwave and oxidation massively lowered the SS content by solubilization/destruction and by improving the settleability of the remaining suspended solids. By clarification, 70% of the volume was recovered as SS-free supernatant. Initially, 40% of the 220 mg-TP/L were ortho-P. With acidification to pH 4, the ratio of ortho-P / TP increased to 60%. With acidification plus MW-H2O2 (0.1-0.3% H2O2), the ratio increased to 90-95%, indicating that the increase in dissolved P was due in equal part to acidification and MW-H2O2. Oxalic acid was added to the supernatant to bind to calcium and prevent re-precipitation of phosphorus with it. A ratio of 2 moles oxalic acid per mole Ca was enough to remove 90% of Ca without binding with Mg. In the final stage, struvite was precipitated without addition of Mg since P was the limiting nutrient. 95% of P was removed from the liquid. No details were given regarding pH adjustment. The product was very pure, between 91 and 97%, with very small amounts of Ca, Na, and K. MW, hydrothermal treatment and pyrolysis have positive effects on transforming the organic matter into a useful liquid product. However, on top of the relatively advanced technological level involved in these pre-treatments, their effect on P dissolution in very limited or absent. Only chemical acidification has a significant and positive effect on P dissolution with as much as 95% TP solubilized into ortho-P. H2SO4 was slightly more efficient than HCl in certain studies and both were equivalent in others. The exact amount of acid was not described in most studies on MW pre-treatment but it seems to have been used in very large excess. A recapitulative table (Table 2) sums up the amount and type of acid used, as well as the ratio of acid to P dissolved during acidification of swine manure and sewage sludge ashes. It appears that ashes from swine manure and sewage sludge give similar results in term of acid to P ratio. Acidification from char or raw manure requires significantly more acid. While no hypothesis was made by Azuara et 37 al. (2013) regarding the higher need for acid during P extraction from char compared to ashes, the reason for higher acid need in raw slurry is quite straightforward: many compounds in raw slurry can act as pH buffer and some of them have been removed from the ashes are transformed into other compounds without buffering capacity. In raw slurry, the ratio of acid to dissolved P is minimized when TP concentration is high and on the contrary the ratio is larger at very low TP concentration, since inorganic P is not the only compound reacting during acidification. Chemical acidification, contrary to more advanced processes, does not require large investment costs, massive energy consumption, and skilled operators. Microwave, hydrothermal carbonization, pyrolysis and incineration are processes suited for very large treatment plants. In rural areas with low population densities, P recovery processes should be kept as simple as possible to be installed on site or regrouping other livestock breeders living nearby. As a result, chemical acidification is the most low tech process, relatively easily applied locally. However, it still constitutes the main post for expenses for struvite recovery from swine manure. In Szögi et al. (2015), after correcting the amount citric acid necessary, acidification represented 76% of chemicals price. (Daumer et al., 2010) found that acidification represented 85% of chemical costs and 65% of the overall process counting investment, energy and maintenance.
BUTYRIC ACID PRODUCTION FROM LACTATE
Several Clostridia (Clostridium butyricum, C. tyrobutyricum, C. paraputrificum) originating from soil contaminating silage are able to consume lactate to produce butyrate, CO2 and hydrogen gas while recovering energy (Equation 10) (McDonald, 1982; McDonald et al., 1973; Oude Elferink et al., 1999). Other microorganisms such as Megasphaera elsdenii, found in the rumen and swine feces (Hino and Kuroda, 1993; Tsukahara et al., 2002) perform the same reaction (Prabhu et al., 2012). Bacteria isolated from human feces, Eubacterium halii and Anaerostipes caccae were able to produce butyrate from lactate and acetate (Duncan et al., 2004) (Equation 11 and Figure 19).
LACTIC ACID FERMENTATION FOLLOWED BY METHANE PRODUCTION
Bo et al. (2007) found that fermentation of kitchen waste led to a conversion of 50%-COD to lactate. The acidified mixture was sent to anaerobic digestion and compared with glucose fed anaerobic digestion. The reactor receiving fermented kitchen waste had a lower methane production due to the accumulation of propionic acid, a known product of secondary fermentation of lactate. These inhibitory effects of lactate on anaerobic digestion were not observed by Daumer (unpublished data). This review of acidogenesis provides useful hindsight regarding biological acidification of pig slurry to dissolve inorganic solid phosphorus. The low pH obtained by Tenca et al. (2011), perfectly sufficient for high P dissolution, indicates that the high alkalinity of swine slurry can be overcome at co-substrate concentration similar to what is commonly used in anaerobic digestion processes. The capacity of LAB to outcompete other bacteria for easily accessible carbohydrates suggests the need to use sugary co-substrates for an optimal acidification process.
Table of contents :
I. PHOSPHORUS, A NUTRIENT AT A CROSSROAD
1. Role of phosphorus in Nature and anthropogenic activities
2. Phosphorus reserve, current and forecasted uses
3. Mitigating measures envisioned to close the phosphorus cycle
II. PROCESSES FOR PHOSPHORUS RECOVERY
1. Agronomic value of struvite
2. Existing processes for phosphorus recovery
a. Technologies for phosphorus recovery from municipal / industrial wastewaters
Recovery from treated effluents
Recovery from the liquid phase of anaerobic digestion
Phosphorus recovery from municipal WWTP digested sludge itself
Pre-treatments of municipal sludge
Conclusion on phosphorus recovery from WWTP
b. Phosphorus recovery from Livestock effluents
Phosphorus recovery from the liquid phase
Pre-treatments prior to phosphorus recovery
III. ACIDOGENESIS
1. Catabolism of organic matter under anaerobic conditions
a. Catabolism of amino acids
b. Catabolism of lipids
c. Catabolism of glucose
2. Lactic acid fermentation
a. Lactic acid bacteria
b. Ensiling process
c. Butyric acid production from lactate
d. Other organic acids produced from lactate
e. Fermentation of swine manure
f. Lactic acid fermentation followed by methane production
IV. CONCLUSION
REFERENCES
OBJECTIVES OF THESIS
CHAPTER 1 DISSOLUTION OF PARTICULATE PHOSPHORUS IN PIG SLURRY THROUGH BIOLOGICAL ACIDIFICATION
I. SUMMARY OF CHAPTER 1
II. ABSTRACT
III. INTRODUCTION
IV. MATERIALS AND METHODS
1. Pig slurry
2. Biological acidification of pig slurry in batch tests using sucrose as co-substrate
3. Analysis
4. Statistics
V. RESULTS AND DISCUSSION
1. Effect of sucrose concentration on pH and fermentation product in slurry 1
a. pH change
b. Evolution of lactic acid, VFAs and sucrose across time
2. Phosphorus, magnesium, calcium and ammonia dissolution processes in slurry 1
3. Dissolution of P, Mg, Ca, N in the other slurries
4. Potential for struvite crystallization
5. Correlating lowest pH with initial sucrose concentration and buffer capacity
VI. CONCLUSIONS
VII. ACKNOWLEDGEMENT
VIII. FUNDING SOURCE
IX. REFERENCES
X. APPENDIX
1. Appendix A
2. Appendix B
3. Appendix C
4. Appendix D
CHAPTER 2 BIOLOGICAL ACIDIFICATION OF SWINE SLURRY: EFFECT OF VARIOUS ORGANIC CO- SUBSTRATES ON PH, ORGANIC ACIDS PRODUCTION AND BACTERIAL POPULATIONS
I. SUMMARY OF CHAPTER 2
II. ABSTRACT
III. INTRODUCTION
IV. MATERIALS AND METHODS
1. Characterization of pig slurry and digested pig slurry
2. Batch experiments
3. Physic and chemical analyses
4. Microbiological analysis
5. Statistical analysis
V. RESULTS AND DISCUSSION
1. Evolution of pH and organic acids concentration in batch tests with sucrose
2. Evolution of pH and organic acid production in batch tests with organic waste as co-substrates
3. Microbial diversity detection using 16S rDNA high throughput sequencing
4. Identification of the dominant species
VI. CONCLUSION
VII. REFERENCES
CHAPTER 3 BIOLOGICAL ACIDIFICATION OF PIG SLURRY USING ORGANIC CO SUBSTRATES: AN EFFICIENT PROCESS FOR PHOSPHORUS DISSOLUTION PRIOR TO STRUVITE CRYSTALLIZATION
I. SUMMARY OF CHAPTER 3
II. ABSTRACT
III. INTRODUCTION
IV. MATERIALS AND METHODS
1. Pig slurries
2. Organic co-substrates
3. Determination of the acidifying power for each co-substrate compared to sucrose
4. Biological acidification of pig slurry in batch tests using complex organic co-substrates
5. Semi continuous reactor
6. Crystallization process
7. Analysis
V. RESULTS AND DISCUSSION
1. Model fitness
2. Dissolution of P, Mg, Ca and N
a. Phosphorus
b. Magnesium calcium and nitrogen dissolution
3. Molar ratios of interest for struvite precipitation
4. Semi-continuous reactor operation and phosphorus recovery
VI. CONCLUSIONS
VII. FUNDING SOURCE
VIII. REFERENCES
DISCUSSION AND PERSPECTIVES .
