Efficiency And Mechanism Of A Soil-wood Biofilter For Treating Piggery Wastewater With Low C/N Ratio

Along with the scale-development of pig farming, more and more swine wastewater is discharged, reusulting in a risk to the receiving environment. It is an imminency for the sustainable development of pig farming to develop new treatment processes that are more economically and effectively, to meet the strict national discharge standard. Sequencing batch reactor(SBR) and upflow anaerobic sludge blanket(UASB) and their combined processes have been used to treat mixed pig manure wastewater after solid-liquid separation. However, their popularization and application are restriced by the complex operating process and high cost of treatment. The technical bottleneck in piggery wastewater treatment is the oxidation of ammonium(NH4+-N) and the nitrogen removal because of the high NH4+-N concentration and low C/N ratio. Based on the performance of three biofillters packed with gravel, wood chips, and mixture of wood chip and soil, respectively, a novel soil-wood chips biofilter(SWBF) was constructed. Its efficiency in removing chemical oxygen demand(COD), NH4+-N and total nitrogen(TN) from manure-free piggery wastewater was evaluated, while the mechanism of pollutant removal was was investagiated.A small-scale SWBF was constructed firstly, and its performance was compaired with a wood chip biofilter(WBF) and gravel biofilter(GBF) in the same scale. The three biofilters were operated in parallel with a same surface hydraulic load(SHL) of 0.2 m3/m2·d, with COD ranged from 181-509 mg/L and NH4+-N from 192.8-802.0 mg/L in the feed. NH4+-N removal of 83.8% and TN removal of 41.3% were reached in the SWBF, remarkably better than that in the WBF and GBF. The result suggested that SWBF was a feasible process for treating manure-free piggery wastewater that with high NH4+-N and low C/N ratio. It was found that the biofilm in the SWBF was more diversiform in shape and microorganism than that in the other two biofilters. The richest ammonia oxidizing bacteria(AOB) in the SWBF resulted in the best NH4+-N removal among the three biofilters. Analysis of CODremoval/TNremoval indicated that the wood chips in the SWBF could work as a carbon source for the denitrification.Effect of SHL on performance of the SWBF was investigated. With influent COD about 266 mg/L and NH4+-N about 467.7 mg/L, the COD, NH4+-N and TN removal averaged 60.7%, 84.2% and 28.6%, respectively, at SHL 0.08 m3/m2·d. When SHL was increased to 0.2 m3/m2·d, the COD and NH4+-N removal decreased to about 58.0% and 68.9%, respectively, with the TN removal increased to about 43.5%. Though the system could perform steadily as before when SHL was farther increased to 0.32 m3/m2·d, the COD, NH4+-N and TN removal decrased to 56.9%, 53.3% and 20.9%, respectively. Based on the results as above, SHL 0.2 m3/m2·d was suggested for the SWBF treating manure-free piggery wastewater to meet the strict national discharge standard.Quality of piggery wastewater varies greatly following the production cycle of pig farming. Thus, influence of wastewater quality on performance of the SWBF treating manure-free piggery wastewater was investigated at SHL 0.2 m3/m2·d. With COD about 152 mg/L and NH4+-N about 175.5 mg/L in influent, an average COD removal of 52.3% and NH4+-N removal of 84.2% was obtained in the SWBF. The COD increased to 61.2%, while the NH4+-N removal decreased to 61.5%, when the influent COD and NH4+-N was increased to about 421 mg/L and 788.7 mg/L, respectively. The best TN removal of 43.5% came up to the influent with a COD of 326 mg/L and a NH4+-N of 568.7 mg/L, when COD and NH4+-N removal in the SWBF averaged 58.0% and 68.9%, respectively. It was found that SHL had a worse influence on the perfermence of the SWBF than that of wastewater quality, while denitrification was more sencitive to the change of SHL than that of ammonium oxidation.Efficiency and technical economy of the SWBF was analyzed, campering to a SBR also treating manure-free wastewater in the same scale. Fed with the same wastewater, COD removal in the SWBF averaged 48.2% at SHL 0.2 m3/m2·d, while 45.6% in the SBR with an aeration time of 5 h. NH4+-N removal in the SWBF reached 78.1%, observably higher than that of 68.7% in the SBR. For engineering with a processing quantity of 200 m3/d, when the effluent satisfied the emission standard(GB18596-2001), investment in capital construction of the SWBF process matched the SBR process. But the direct operational cost of the SWBF process was lower than that of the SBR process by 63%. The results showed that the SWBF was better in technical economy and easier to management than the SBR.By monitoring the pollutant concerntion in the soil from 10, 30, 50, 70, 90, 110 cm of biofilter under the stable state, variation of major pollutants along the filter bed depth was investigated. The results showed that all most of the organics in the wastewater coud be degradated above the depth of 30 cm, and remarkable accumulation of septic matter from the packed wood chips was observed in the depth of 50 cm. NH4+-N concentration in soil presented a peak at the bed depth of 30 cm or 50 cm following the change of SHL, while nitrate performed an upward tendency from top to bottom with a degressive accumulation. And then, to understand the conversion mechanism of NH4+-N in the SWBF, functional relationship between effluent NH4+-N concentration and depth of the filter bed was constracted. Enffluent NH4+-N concentration is the function of the depth of the filter bed. The function based on semiempirical model is an ideal tool for designing the depth of SWBF by enffluent NH4+-N concentration required in wastewater treatment engineerings.The biological mechanism for pollutant removal in the SWBF was investigatedbased on structure elucidation of microbiologic population in the system. Though dissolved oxygen(DO) in the filter bed was degressive along the depth, regular distribution of anaerobic microenvironment was found in the SWBF. Function regions coud be identified in the filter bed. Thogh bacterial species was the most abundant on depth 70 cm, lignocelluloses degradation bacteria assembled in the region from 0-50 cm, while bacteria degradating terpene compounds were clustered in the region from 90-110 cm. Decomposition of the wood chips in the filter bed was an inner carbon source for heterotrophic denitrification bacteria. It was found that nitrogenous metabolism in the region from 10-50 cm was diversity, inclouding ammoxidation, nitrite oxidation and heterotrophic denitrification. In the region from 70-110 cm, the denitrification was dominated by heterotrophic reduction of nitrate and anammox.