Although the BOD levels of the effluents from stabilization ponds are lower than the standard effluent of Thailand (20 mg/L), both nitrogen and phosphorus concentrations are significantly high. Stabilization ponds currently in use do not adequately decrease both nitrogen and phosphorus concentrations. The excessive N and P from stabilization ponds should
not be discharged directly to the environment. TKN (organic nitrogen and ammonium nitrogen) can increase the BOD value or decrease the DO value in water resources and phosphorus is a major nutrient that enhances eutrophication. However, the regulation of effluent nutrient standards in Thailand for nitrate and phosphorus are 10 mg N/L and 2 mg P/L, respectively. These standards are applied to only some areas in Bangkok. The phosphorus standard is about 10 times higher than the standards of Australia and United States of America. Environmental scientists need to develop a suitable standard for individual effluents, domestic, industrial, and agricultural wastewaters in Thailand. A possible solution for increasing the efficiency of phosphorus removal in stabilization ponds is by adding other treatments at the end of stabilization ponds, such as wetland or a high efficiency phosphorus adsorption unit. It would be beneficial to find a material which would effectively adsorb phosphorus while providing growth medium for nitrogen-removing bacteria. Based on the literature review, Zhu et al. (1997) reported that Utelite from Utah, U.S.A could adsorb P at 3.46 g P/Kg. Sakadevan and Bavor (1998), moreover, showed that blast furnace slag from Australian Steel Mills Limited could adsorb P at 44.2 g P/Kg. Jenssen, et al. (1991) reported that Leca from Sweden had an adsorption capacity of 1.62 g P/Kg. Ex-un et al. (in prep.) determined that natural zeolite from Thailand could adsorb P at 0.10 g P/Kg. These four substrates provide very high P adsorption capacity. It is likely that P adsorption units could be applied to domestic sewage from septic tanks too. As we know, disposal of domestic sewage in each house uses a septic tank but this system cannot be used to treat both P and N substrates. Improving the removal of both organic matter and nutrients from decentralized wastewater (onsite treatment) is very important and urgent. However, any new methods (water reuse and recycling for agriculture or some industries) should have low operational and maintenance costs if they are to be applied in rural areas in Thailand. The best way to remove both N and P is by combination of biological and chemical treatments. Material to highly adsorb phosphorus while
providing a growth medium for nitrogen removing bacteria should be determined for use in the field.