The Role Of Mg Residue In Affecting Ammonia In Cattle Manure Composting And Biogas Slurry Treatment

With the rapid expansion and intensification of livestock industry in recent years, the amount of animal waste produced is very considerable, containing a great deal of N, P, organic compounds and pathogens. Such wastes can cause severe damage to environment and human health if discharged directly into the environment. Currently, composting of solid waste and biogas fermentation of the wastewater are the main and effective ways to deal with such wastes.Aerobic composting is the main way to detoxify, reduce and recycle animal manure. Traditional composting process usually leads to a large amount of N volatizing, which not only reduces the fertilizer efficiency of the compost product, but also produces stink, influencing the nearby environment. It’s shown that Mg salt can act as a stabilizer to cut down the N loss in composting process, but this method is limited by the high chemical cost.Using anaerobic fermentation to deal with wastewater from livestock industry can reduce the COD and BOD, and can recycle biogas resource at the same time. However, the biogas slurry produced by the digester has the feature of high ammonia and low C/N, which can hardly accept bio-treatment directly. Using magnesium ammonium phosphate（MAP） precipitation to remove the ammonia has the advantage of rapid and efficient, but the Mg and P concentration in the biogas slurry is much lower than N, so that large amount of Mg salt and phosphate are needed and cause high chemical cost, which also limiting the promotion and application of MAP precipitation.Mg salts have broad application prospects in treating animal wastes. The traditional Mg salts are generally Mg Cl2 and Mg SO4, which have a high cost. Mg residue is the byproduct of Mg O fume desulfurization and the main composition are Mg SO4 andMgSO₃. Currently, Mg residue is mainly discarded in China, which is a waste to resources and may result in secondary pollution.Based on that, this thesis mainly studied the feasibility of using Mg residue in treating waste residue from livestock industries, including the nitrogen retention effect of Mg residue in cattle manure composting and the application of Mg residue as an economical magnesium salt in MAP precipitation to pretreat the pig farm biogas slurry.（1） High-temperature phase is the main phase of nitrogen loss in composting. Thermostat incubator was used to simulate the high-temperature phase in composting, and KH2PO4, KH2PO4+Mg SO4 and KH2PO4+K2SO4 were dosed into the compost to study the nitrogen retention effect of different ions in MAP method.（2） Based on the previous experiment, Mg residue was applied as the stabilizer to study the nitrogen retention effect in comparison with Mg SO4.（3） Germination index（GI） of radish seeds was used as the indicator to examine the effects of Mg residue to the quality of compost products.（4） Mg residue was applied as an additive in MAP precipitation to pretreat the pig farm biogas slurry, in order to alleviate the inhibiting effect of ammonia to microorganisms in the consequent bio-treatment process, and treatment efficiency and chemical cost were assessed compared with Mg Cl2. Results showed that:（1） After 15 d high temperature incubation, 3 treatments, 2%（mass ratio, same below） KH2PO4, 2% KH2PO4+5% Mg SO4 and 2% KH2PO4+3.5% K2SO4, had the nitrogen fixing rates of 10.11%, 20.19% and 11.77% relatively, compared with control. That indicated KH2PO4 addition could reduce the amount of N volatilized during the high-temperature phase of composting. Compared with single KH2PO4 addition, adding Mg SO4 could promote the nitrogen fixing effect to a large extent, while adding K2SO4 had no significant effects. Mg ions have a nitrogen fixing effect in composting, and the effect could be enhanced by P source, for which MAP crystal formation could be a possible explanation.（2） Mg residue showed a similar nitrogen retention effect with Mg SO4 and the effect was promoted with the increment of Mg residue adding amount. The nitrogen fixing rates were 6.15%, 10.13% and 19.51% respectively with the addition proportion of2%, 4% and 8% Mg residue. Adding Mg residue and Mg SO4 both reduced the compost p H significantly and promoted the ammonia concentration. The ammonia concentration in 2%, 4% and 8% Mg residue addition treatment increased 6.85%, 57.55% and 77.11%, respectively, compared to control. While in 2% and 4% Mg SO4 addition treatment, the ammonia concentration increased 14.47% and 54.81%, which indicates that using Mg residue as a substitute to Mg SO4 as the nitrogen fixing agent in composting has certain environmental and economic benefits.（3） The GI of the control was 113%, while in other treatments, the value was ranged from 50% to 80%., indicating that the addition of Mg salt to some extent decreased the GI. 3 aspects may explain this result. Firstly, the existence of stabilizer could promote the electrical conductivity（EC） of the compost. At the end of the high-temperature phase, the EC values of control, 2% Mg SO4, 4% Mg SO4, 2% Mg residue, 4% Mg residue, 8% Mg residue were 0.97, 1.32, 1.69, 1.44, 1.91 and 2.72 ms·cm-1 respectively. Secondly, addition of Mg salt increased the ammonia concentration in the pile significantly and thus germination was inhibited.（4） 3 main factors in MAP precipitation, p H, HRT（hydraulic retention time） and molar ratio of P:Mg:N were studied to reduce the ammonia concentration in the pig farm biogas slurry from 440.39 mg/L to below 200 mg/L and the best treatment condition was assessed. Results showed that when the p H was 9.5, HRT was 10 min and the molar ratio of P:Mg:N was 0.6:0.7:1, NH3-N, PO43--P and Mg2+ residual amount in the pig farm biogas slurry was 183.20 mg·L-1, 3.64 mg/L and 67.17 mg/L respectively. Compared to Mg Cl2 treatment, 29.17% of chemical cost was saved. SEM-EDS and XRD analysis to the precipitation showed that it contains MAP.In conclusion, adding Mg residue into cattle manure compost at high-temperature phase could reduce the nitrogen volatizing effectively and had a good nitrogen fixing effect. In pretreatment of biogas slurry, adding Mg residue could recycle the N and P in the slurry and enhance the biodegradability. Adding Mg residue could also cut down the chemical cost in both composting and slurry pretreatment and thus had certain environmental and economic benefits.