Experiments on livestock and poultry farming wastewater filtered by negative pressure

Wastewater treatment has been one of the restricting factors in the development of livestock and poultry industry, due to the difficulty and high cost. Furthermore, there is serious pollution to soil, air and groundwater, if the farming wastewater is discharged without any treatment. Current biogas engineering can cause a heavy load and high blocking of wastewater. Fortunately, crop straw can serve as filter material for the filtration pre-treatment of livestock and poultry farming wastewater. The suspended solids can also be removed to effectively improve wastewater treatment. However, it is very necessary to promote the filtering speed and utilization rate of straw at present. In this study, the filtering project was proposed to solve these challenges using negative pressure. A series of tests were also carried out on the livestock and poultry farming wastewater. Firstly, the test platform was constructed to filter the farming wastewater using negative pressure, which was mainly composed of a filtration device, control box, sewage pump, vacuum pump, and pressure sensor. Specifically, the filter device was used to form the filter layer built by crop straw and the enclosed space required for filtration; The sewage pump was to pump the wastewater into the filter device; The vacuum pump was to form a vacuum degree in the filter device; The pressure sensor was to detect the air pressure in the filter device, and then control the vacuum pump on (or off) together with the PLC controller in the control box, in order to ensure the pressure stable within the set range. Secondly, the test factors were selected as the thickness and compaction density of the filtration layer, as well as the filtration pressure. The test range of each factor was then determined by the single-factor experiments. Moreover, the quadratic regression experiment was implemented to determine the filtration parameters of the optimal performance on filtering by negative pressure. The thickness and compaction density of the filtration layer and the filtration pressure were 10 cm, 130 kg/m³, and 30 kPa, respectively. Design-Expert 12 software was used to analyze the influence of the interaction among factors on the removal performance of wastewater. It was found that the removal rates of total solid (TS), volatile solid (VS), chemical oxygen demand (COD), and NH4+-N were 40.47%, 45.07, 13.56%, and 11.75%, respectively, indicating effective removal of solid pollutants in wastewater. Then, comparative tests were conducted on filtration between negative pressure and gravity under the same conditions. It was found that the amount of treated wastewater filtering by negative pressure was four times that of gravity, and the duration was only 1/5, indicating the higher filtration speed and utilization rate of straw. Finally, a comparative test was carried out on the anaerobic fermentation of wastewater. The wastewater filtering by negative pressure and gravity shortened the fermentation period by 30%, compared with the untreated wastewater. Gas production was stable without any fluctuation, indicating the reduction of the load during anaerobic fermentation. In summary, it is feasible for the crop straw as filter material to filter the livestock and poultry farming wastewater by negative pressure. The finding can provide technical support to the filtration equipment for the livestock and poultry wastewater.