| With the improvement of people’s living standard, the demand for meat has increased rapidly. China is a country with a large population, and has a strong demand for pork since ancient times. In this context, very small-scale, decentralized farming mode can not meet the market demand for pork. The large-scale, intensive production mode has become the general trend. The new production mode caused the issue of discharging large amount of concentrated swine wastewater. After anaerobic digestion (AD) of the swine manure for biogas production, the remaining slurry still contains high N, P and other organic nutrients. The purpose of this study was to treat and utilize the remaining manure slurry after anaerobic fermentation using Chlorella vulgaris, hoping to purify the slurry as well as producting Chlorella algal biomass in low cost. Therefore the wastewater resource can be fully utilized and treated. However, this slurry contains high N, P and organic matter and tends to inhibit the growth of chlorella. In this research, various reasonable and effective pretreatment methods were explored to ensure their ability to support slurry Chlorella sp growth. The main pretreatment methods selected included dilution, iron carbon micro electrolysis, flocculation, physical adsorption, biological membrane method and magnesium ammonium phosphate precipitation aimed at high ammonia nitrogen. The main results are as follows:1. The microalgae which adapted well in the AD treated swine manure slurry were isolated from swine farm. The strain was identified as Chlorella sp. The growth condition, main composition of algae and the nutrients removal efficiency were investigated. The results showed that the Chlorella sp had a good wastewater nutrient removal effect for the slurry. The wastewater nutrient removal efficiency of COD, TN, ammonia nitrogen and TP in not diluted AD-treated slurry were 78.32%,59.51%, 98.61%,97.90% respectively by second culture method. The harvested Chlorella sp biomass contains high protein and oil. The concentration of the nutritional components of Chlorella sp powder were different when cultivated with different level diluted slurry.2. The removal efficiency for the main heavy metals and P were very high, however the N removal rate was very low when using the iron carbon micro electrolysis pretreatment of the slurry. The biogas slurry appeared to have high ammonia nitrogen and low phosphorus, double negative state for the Chlorella sp cultivation, resulting in a decrease in the growth rate of Chlorella sp, the slurry nutrient removal rate is not good either. Therefore, the pretreatment method is not suitable for the pretreatment of the slurry for microalgae production and nutrient removal.3. The optimal addition amount of ferric chloride is 2g/L when used for the pretreatment the slurry. The removal efficiency of TN, TP and COD were 11.04%, 97.01%,53.91% respectively. However, it can be seen that the ferric chloride pretreated slurry was no longer suitable for culturing Chlorella sp, since after treatment, the slurry had the characteristics of high N, low P and containing large amounts of flocculant Fe3+. As for microbial flocculant poly glutamic acid, when additive amount was 2g/L the removal efficiency of TN, ammonia nitrogen, COD and TP were 20.28%,25.77%,20.81%and 42.31%, respectively. In Chlorella sp cultivation stage, different amount of added poly glutamic acid had no effect on Chlorella sp growth rate and nutrients removal efficiency.4. Zeolite treatment group had the best pretreatment effect on the AD-treated swine manure slurry among the three physical adsorption agents, zeolite, foam brick and red clay. The removal efficiency of TN, ammonia nitrogen, TP and COD were 4.76%,24.49%,18.95% and 4.42%, respectively. In Chlorella sp cultivation stage, different treatment materials had no significant difference. Without activation, the zeolite, foam brick and red clay were not suitable as physical adsorption agents for the slurry pretreatment. There is no practical value even if the best activated zeolite is used for pretreatment, since cost is too high for the extremely huge amount of the slurry. Therefore, the physical adsorption method is not suitable for the slurry pretreatment for Chlorella sp cultivation.5. Immobilized nitrifying bacteria could make use of sugarcane bagasse’s nutrients and grow and reproduce rapidly on the bagasse. The results showed that the slurry purification ability of the experimental group was obviously higher than that of the control group. During the biological membrane treatment stage, the removal efficiency of TN, ammonia nitrogen, TP and COD were 89.18%,98.46%,98.15% and 63.94%, respectively. Although, after the biological membrane pretreatment, the residual amount of TN, ammonia nitrogen, TP and COD were only about 57.31mg/L, 2.25mg/L,0.76mg/L and 178.29mg/L, respectively. Therefore the pretreated slurry is no longer suitable for cultivation of Chlorella sp due to a serious shortage of nitrogen and phosphorus.6. Experiments were also carried out to investigate main factors influencing the magnesium ammonium phosphate precipitation in order to reduce the concentration of ammonia nitrogen in the slurry. The results showed that the optimal precipitation conditions of magnesium ammonium phosphate were magnesia addition amount of 800 mg/L, granularity size of dolomite and its addition amount of 80-120 mesh and 2000mg/L, respectively, pH 10.0, reaction time 100min, and stirring speed 150r/min, and reaction temperature 25 ℃. |
PDF Full Text Request