Research And Application Of UARI System In Domestic Sewage Treatment

Centralized wastewater treatment plant (CWWTP) is widely been used in large cities. However, it is not only costly in both construction and operation, but generates large amounts of sludge which must be properly disposed of, and releases odorous compounds to the surrounding environment. Further, in countryside, small towns and suburban colonies, the use of CWWTP is largely limited by the difficulties in wastewater collection. Thus, there is an urgent need for the development of cost-effective and environment friendly decentralized wastewater treatment technologies.Underground infiltration wastewater treatment system has the characteristics of (1) very low cost of operation; (2) releasing little smells; (3) free of noise; (4) slightly influenced by climate. Moreover, its surface ground can be used as vegetation land, parking lot, etc. Therefore, it is a low-cost system that can be built in urban and separate colonies and, thus, has widely been used abroad. However, the hydrologic loading permitted in the traditional system is very low (1-4 cm/d), which largely limited the used of this kind system in more densely populated area.Based on the recognition that clogging by organic mat and short of oxygen in the leaching field are the main factors limiting the hydrologic loading, a high hydrologic loading underground artificial rapid infiltration (UARI) system was developed. The main work of this study includes (1) a pilot scale test of the UARI composite system; (2) mechanisms for the transformation and removal of NH₄⁺-N and TP during wastewater infiltration; (3) influence of microbes and enzyme activity on the operation of the leaching system; (4) numerical modeling of particulate organic matter accumulation in, and estimation of pollutant loading capacity of, the leaching system. The major results and conclusions are as follows.(1) The pilot system is buried underground and comprises an anaerobic pre-treating unit to remove suspended particles followed by a leaching field. The removing rate of COD_Cr, BOD₅, TN, NH₄⁺, TP and TSS was up to 76.7-89.1%, 83.3—92.5%, 50.3— 62.1%, 65.2—79.6%, 75.4—90.1% and 77.0—91.3%, respectively, at a hydrologic loading of 40 cm/d, and the content of each pollutant was less than their respective limiting values for the water discharge from wastewater treatment plants.(2) In the leaching field, most of the NH₄⁺ is initially adsorbed by soil granules during the cyclic wastewater infiltration, and is nitrified during the subsequent dry period. The nitrification of NH₄⁺ from wastewater was mainly influenced by oxygen supply and the growth of relevant microorganisms.(3) There is a general downward decrease of phosphorus content along the soil column in which wastewater infiltration experiment was performed, and the phosphorus in the soil column was dominated by Al-bound, Fe-bound and Ca-bound forms. Hence, the phosphorus from wastewater would initially be adsorbed by soil granules, but subsequently combined with metals to form phosphate precipitates.(4) The microbe community in the soil column of wastewater infiltration experiments would be dominated by bacillus, epiphyte and actinomyces. Meanwhile, anaerobe was abundant in the soil column except for the uppermost layer where high oxygen supply is expected, with the concentration of nitrobacteria and denitrifying bacteria being highest in the soil layer which is made by mixing sand and clay.(5) According to the addition-biodecomposition-bioproduction model of organic matter, the "sludge balance" would be achieved in the leaching field, when the amount of newly added and bio-synthesized organic matter equals to the amount of organic matter that is simultaneously consumed by biodecomposition. If the accumulative amount of foreign organic matter in the infiltration column equals to its capacity of foreign organic matter when the hydro-conductivity is retained, the maximum pollutant loading can be defined. A numerical model for calculating the accumulative foreign organic matter was developed and its parameters were estimated by the regression analysis of experimental data. According to the model calculation, the infiltration soil will never be clogged by the accumulation of organic matter when the hydrologic loading of wastewater (COD=300 mg/l, TSS=70 mg/l) is as high as 75 cm/d.