The Treatment Of Digested Swine Wastewater By Vertical Flow Constructed Wetlands

Anaerobic digestion technology has been widely applied in the treatment of swine wastewater for its advantages of directly high concentrated organic matter treating, less sludge producing, energy saving and biogas generating. However, concentrations of COD, nitrogen and phosphorus in digested effluent remain high. The digested effluent should be post-treated before being discharged, if there were not enough lands around pig farms to receive digested effluent. This paper has studied the treatment of digested swine wastewater by vertical flow constructed wetlands (VFCWs) strengthening technology. Research conclusions are as follows:1. Packing material screening of VFCWs. This experiment compared ammonia nitrogen removal efficiencies of five different types of substrates-Minjiang sand, Tuojiang sand, Qingyijiang sand, zeolite and activated carbon- by means of static adsorption experiment. The Minjiang sand with the highest cost-effective performance was screened to be the packing material of this experiment, based on its high ammonia removal capability, low purchase costs and low transportation costs. The adsorption experimental results showed that ammonia nitrogen removal efficiency of the sand increased as contact time and dosing quantity increased. The data obtained in these experiments were in accordance with Freundlich and Langmuir isotherms. Maximum adsorption capacity of sand calculated from Langmuir Equation was 0.418 mg·g-1, which has little differences from measured adsorption capacity.2. Comparative study on the system with addition of sludge and with addition of artificial zeolite. In this study, three lab-scale VFCWs systems were designed. Results indicated that the effluent pH values of 1# (only packed with sand as control),2# (added aerobic nitrification sludge to the sand),3# (added artificial zeolite to the sand) decreased from 7.93,7.80, 8.60 to 7.54,7.51,7.52, respectively. Concentration based average removal efficiencies for 1#,2# and 3# were COD,69.9%,73.1% and 80.4%; NH4-N, 72.3%,76.2% and 86.7%. The total NH4-N bio-conversion efficiencies of three systems were 66.4%,67.5%,86.1%. Additionally,3# could maintain at least 72d for total phosphorus concentration to meet the discharge standard of pollutants for livestock and poultry breeding (GB 18596-2001), whereas 2# could only maintain 19d as same as 1#. Based on these findings, using VFCWs with a local sand as packing material with addition of artificial zeolite could enhance the performance of the system.3. Investigated effects of pollution loading and hydraulic loading on VFCWs. When the ammonia loading was less than 21.1 gNH4+-N/(m3-d), the effluent concentration of ammonia could meet the discharge standard of pollutants for livestock and poultry breeding(GB 18596-2001). On condition that the effluent TP concentration of high, medium, low phosphorus load systems could meet the standard, high system could remove more phosphorus. Under the same pollution loading condition, the hydraulic loading had a little effects on the removal of ammonia. Concentration based average removal efficiencies for low, medium and high hydraulic loading were 94.1%,92.0% and 90.7%. Ammonia-nitrogen removal amount were 21.6g NH4+-N,21.1g NH4+-N and 20.7g NH4+-N. However, the hydraulic loading had great effects on the removal of phosphate. Concentration based average removal efficiencies for low, medium and high hydraulic loading were 80.5%, 61.7% and 38.2%. Phosphate removal amount were 950mgTP,758mgTP,548mgTP, respectively.4. Clarify the effect of biological and nonbiological factors on VFCWs to remove nitrogen and phosphorus. Nonbiological factor-adsorption effect of substrate played an important role to remove ammonia nitrogen. At the first stage, the influent and substrate of 1# system were sterilized, which could still maintain high ammonia removal of 95.9%. Biological factor-microbial activity could keep the system have strong impact resistance on load capacities. When the ammonia loading was improved, ammonia removal efficiencies for 1# and 2# system with substrate sterilized decreased mostly, which dropped by 25.1%,13.8% respectively, compared with 1.2% of 3# system which had the microbial activity. In constructed wetlands systems, microorganisms mainly converted NH4-N that had been adsorbed onto the surface of the sand. At the start-up phase of the experiment, only a small amount of microorganisms were in these systems, so the removal of NH4-N was mainly due to the adsorption of the sand. In this stage, the conversion of NH4-N into NO2-N and NO3-N was limited by the quantity of microorganisms. As microorganisms accumulated, microorganisms began to convert adsorbed NH4-N into NO2-N and NO3-N increasingly, including the NH4-N adsorbed at the start-up phase. Phosphate removal mainly depended on nonbiological factor-adsorption effect while biological factor-microbial activity had a little effect on the removal.5. Study on the oxygen source and the supply mechanism of VFCWs. Oxygen mainly came into system with influent while oxygen came from the bottom outlet could be neglected. Water level could affect the treatment of digested swine wastewater by VFCWs. When water level was high, the system would be anaerobic condition, which went against ammonia nitrogen conversion. However, high water level would make complete contact between wastewater and substrate, which was beneficial to increase substrate adsorption of TP. Additionally, overturning the substrate regularly would be conducive to phosphorus removal.6. Pilot scale research on integrated constructed wetland for digested swine wastewater. Pollutant removal ability of the preceding processing unit was higher than the post-processing unit. Treatment effect was greatly affected by temperature. The mismanagement led to large changes of influent concentration, uneven distribution of water and the significant variation of annual effluent concentrations for COD, NH3-N, TP concentrations changed greatly.