When ammonia level in an anaerobic digester (AD), it can cause toxicity to methanogens and result in low biogas production. At the same time, ammonia along with phorphorus is a good nutrient for cropping, so it should be extracted from digested material before the latter is disposed of. MgO has been applied for removing ammonia and phosphorus in an AD reactor by forming struvite. In general, ammonia level is much higher than phosphorus level, often phosphate is added into AD reactors. As a result, a large amount of struvite (NH4MgPO4.6H2O) is formed to often cause scale issue. In addition, it is more costly because addition of magnesium and supplemental phosphorus is required. In this study, a simple strategy simultaneouly to reduce ammonia toxicity and to recover nutrients (N and P) in the mixed liqour of an AD reactor in which sewage slujdge, food waste, and swine slurry are co-digested; the solid content of the feed to the AD system was 7% (w/w). Phosphate level in the effluent from the AD reactor was monitored to determine the dose of magnesium oxide which was added to for struvite formation. Through a serious of biochemical methane potential (BMP) tests, it was found that NH4+-N of &gt; 2000 mg/L could result in decreased methane production. In the study with a continuously fed two-stage AD system (hydraulic retention time (HRT) of the first stage for acid genesis was 3 d , and HRT of the second stage for methane generation was 20 d), NH4+ level in feed ranged 2500 mg N/L to 3500 mg N/L while PO43- level was 500-700 mg P/L. However, during the AD process, NH4+ and PO43- levels increased to 5000-7500 mg N/L and 1500-2500 mg P/L, respectively. Therefore, the effluent PO43- concentration was monitored once a day to determine MgO dose to form struvite as much as all the phosphate in the reactor was removed. As a result, about 20% increase of methane gas was observed comparing to the system without MgO addition. About 1/3 of NH4+ or NH3 in the system could be removed from the mixed liquor. Interestingly, in the effluent from the system with MgO added, only trace level of PO43- was detected possibly because most PO43- in the system had been removed through struvite formation; the struvite can be separated from the effluent and reused as a fertilizer. No distinct scale formation in the tube of inside the reactor was not observed.

Simultaneous reduction of ammonia toxicity and recovery of nutrients in mixed liquor of anaerobic digester

255th National Meeting (2018)

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