Advanced purification of carbonization wastewater by activated sludge treatment

Abstract

A full scale activated sludge plant has been developed treating 960 m³ of carbonization wastewater daily. Results and process parameters from the first three years of operation are described. In spite of intense physical‐chemical pretreatment, the carbonization wastewater must still be diluted by 50% prior to biological processing due to the presence of inhibitory organic compounds. The activated sludge plant consists of four serially connected aeration tanks. The influent is distributed following a step load regime. Other specific process characteristics are: pure oxygen aeration, high mixed liquor volatile suspended solids (MLVSS) levels of 10–15 kg MLVSS/m³, and a high sludge age of 100–150 days. The first aeration tank is kept anoxic, making it possible to implement combined nitrification and denitrification. Average reductions in chemical oxygen demand (COD) and total reduced nitrogen (NH₄ ⁺ and CNS^‐) were 90% and 77%, respectively. Average final effluent levels obtained, after a four month startup period, were: 308 mg COD/L, 1.9 mg CNS^‐/L, 4 mg NH⁺ ₄/L, 76 mg NO₃ ^‐/L, 15 mg S₂O² ₃ ^‐/L and 0.3 mg CN^‐/L. Improving the effluent quality by direct flocculant addition prior to sludge sedimentation additionally reduced the average discharged total COD to 254 mg COD/L. This corresponds to an average of 0.14 kg total COD discharged per ton of cokes produced. The most significant result was the extended removal of the high nitrogen load, although nitrification was the process most sensitive to inhibition. The maximal total ammonium load (NH⁺ ₄ + NH⁺ ₄ liberated by CNS^‐ degradation) which did not inhibit nitrification and still guaranteed low ammonium effluent levels (less than 5 mg NH⁺ ₄/L), was found to be 0.015 kg NH⁺ ₄/kg MLVSS day. The maximum allowable sulfide concentration present in the anoxicdenitrification compartment should not exceed 0.2 mg H₂S/L to ensure stable continuous nitrification when the process is run at the previously mentioned maximal ammonium load. It can be concluded that a biotechnological process technology has been developed which is capable of treating carbonization wastewater and of ensuring excellent COD and nitrogen removal.

Notes

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