Effect of solid-liquid separation on removal of total solid and pollutants from pig manure wastewater before anaerobic digestion

The content of contaminants in manures wastewater is much higher than urban domestic sewage, which leads to a high and sophistic biochemical treatment and a high cost of wastewater treatment. The high concentration of pollutants in manures wastewater mainly comes from the dissolution or decomposition of solid feces. Therefore, a solid-liquid separation is used immediately after the production of manures wastewater, which can effectively separate the solid substances from the wastewater. As a result, the content of pollutants in the wastewater also can be effectively reduced. In this study, a solid-liquid separation test was carried out to investigated the ability of centrifugation in removal of the total solid (TS), and reduction of chemical oxygen demand (COD) and the elements of nitrogen (N) and phosphorus (P) from pig manure wastewater. Besides, the economic benefits of centrifugation before anaerobic digestion (AD) treatment were also analyzed. The result showed that the centrifuge separation carried out at 1 000-3 000 r/min for 0.5 minute can effectively remove the TS, COD and N, P elements from pig manure wastewater (TS=7.3 g/L), and the removal rates of them were 50%-65%, 45%-55% and 30%-50%, respectively, and the removal rate increased as the increase of centrifugal speed from 1 000 to 3 000 r/min In addition, the separation efficiency of N and P compounds were lower than that for dry matter, which was similar to the previous reports. This was mainly because the N and P compounds were easier to dissolve in water or be decomposed by microbes than carbohydrate, the major component of solid. The biodegradable solid reduction rate (BSRR) was defined as the reduced percentage of methane potential of manure wastewater after centrifugation, which can be calculated by subtracting the ratio of waste water methane potential before and after centrifugation. The BSRR can be used as the instruction for the adjustment to the construction volume of anaerobic tank and the hydraulic retention time (HRT) in anaerobic digestion (AD) process. According to the result of biochemical methane potential (BMP) test, it was found that the content of biodegradable materials can be reduced significantly by centrifugal separation, the maximum BSRR of 57.6% was observed with 3 000 r/min, which would be able to greatly save the construction volume and HRT in AD process. For a pig farm with ten thousand pigs, the amount of wastewater produced per day was about one hundred ton, the combined treatment of centrifugation and AD can save 45% of the wastewater treatment cost compared with the sole AD mode. If the incomes of biogas and separated solid manure were taken into consideration, the total overall comprehensive income of centrifugation followed AD and the sole AD mode was positive, and the former was 50% higher than the later. However, in practice, the biogas and solid manure are hard to get a profit due to many complex factors. Therefore, the meaning of solid-liquid separation before biochemical treatment would be more important. Moreover, the costs of other treatments after AD process and energy consumption for heat preservation of pool body also can be greatly reduced due to the partial reduction of N, and P contaminants in the centrifugation process and the smaller biochemical pools matched with the centrifugal wastewater. This study can provide new design ideas and references for large and medium-sized farms to save the treatment cost of wastewater and construction investment. © 2018, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.