By Lee Yu Wee
Trienekens (Sarawak) Sdn Bhd manages and operates the Kuching Integrated Waste Management Park – an integrated facility for the treatment and disposal of both municipal and hazardous waste. Located in Kuching, Sarawak; components include a Level 4- Sanitary Landfill, a Secure Landfill, a Leachate Wastewater Treatment Plant, two Incineration Plants, and a full-service laboratory, among others.
The R&D team of the Facility’s Leachate Wastewater Treatment Plant (LWTP) recently made a breakthrough when they successfully improved contaminant removal process efficiencies for mature leachate wastewater, using an unconventional method of adding molasses to the mix.
The LWTP treats leachate generated from the Park’s Sanitary and Secure Landfills. As characteristics of leachate wastewater varies according to the age of a landfill’s cells; there are generally three types of generated leachate according to different phases, namely fresh/new leachate, intermediate leachate and mature/old leachate. The category of leachate is identified based on its Biochemical Oxygen Demand and Chemical Oxygen Demand ratio (BOD:COD) where <0.2 is indicated as matured leachate wastewater, while intermediate and fresh leachate have BOD:COD ratios of 0.2-0.4 and >0.4; respectively.
The Kuching Integrated Waste Management Park’s Leachate Wastewater Treatment Plant is fully automated and continuously monitored via Programmable Logic Controllers (PLC) with a daily treatment capacity of 1000m3.
Treatment of old leachate is very challenging due to the extreme COD concentration deviation after the biological treatment process which typically ranges from 600 mg/L to 2,000 mg/L. The COD cannot be reduced further biologically although all the treatment factors such as COD or BOD loadings, pH, oxygen level and etc. are controlled accordingly. It was observed that BOD in old leachate is very low compared to new leachate. In addition, nitrite concentration is proportional with the COD concentration. Therefore, the de-nitrification process was incomplete due to high access of nitrite in the effluent.
Nitrite is generated during the nitrification process in biological treatment stages, where it is converted to nitrogen gas through de-nitrification process in the Plant’s anoxic tank (with the presence of naturally occurred carbon source in an anoxic condition). At the time of study, BOD/AN ratio were approximately 0.6:1 which was lower than the recommended/expected typical value reported in available literatures on de-nitrification; at 3:1. Lower ratio shows that the carbon source or BOD loadings were deficient due to nitrite not being converted completely to nitrogen gas in the anoxic tank. According to Charles G. et al., high concentrations in nitrite will inhibit the de-nitrification process. Hence, nitrite is accumulated and de-nitrification process suppressed. This condition is normally known as ‘nitrite-lock’ where the portion of incomplete conversion of nitrite will interfere with the COD concentration as ‘inorganic-COD’.
Increasing the BOD loadings in the anoxic tank required carbon sources; and with many different types of additional carbon sources in the market, the Trienekens’ R&D team experimented with several types (in particular; molasses, sugar, methanol, acetic acid and wheat flour) in the Plant’s anoxic tank. All the trialled supplementary carbon sources were able to improve the de-nitrification process, showing a reduction of nitrite concentration between 90% – 95% after dosing. Among all though, molasses and sugar were the most time effective in nitrite removal compared to methanol, acetic acid and wheat flour. Operationally, sugar had handling constraints as its solid physical state required mixing and diluting before dosing was possible, as well as being a commodity-controlled item. Due to its relatively low cost and uncomplicated handling compared to the other trial sources, molasses was found to be the most appropriate supplementary carbon source for improving the de-nitrification process. Consequently, COD concentrations also reduced significantly due to less ‘inorganic-COD’ influences on the effluent quality. In addition, using this more traditional treatment method to treat mature leachate wastewater has proven to be more environmentally-friendly besides also being cost effective; benefiting both CAPEX and OPEX compared to other more popular contemporary treatment alternatives, such as Ozonation or Fenton processes.