Effective treatment of leachate concentrate from waste incineration plant by combination of coagulation and direct contact evaporation

Abstract

In order to verify that coagulation as pre-treatment can reduce the temperature of the hot air used for direct contact evaporating the leachate concentrate (LC) and low-grade waste heat such as exhaust steam in the waste incineration plant can be used to evaporate the LC. The supernatants after coagulation using polymerized ferrous sulfate (PFS), polymeric-aluminum (PAC), polymeric silicate aluminum ferric (PSAF) and poly-aluminum ferric chloride (PAFC) as coagulants were further treated in a lab-scale direct contact evaporation system. The results showed that the best performance with removal efficiencies of COD and NH3–N of 58.70% and 29.09% was achieved after coagulation when PAFC dosage = 15 g/L, PAM dosage = 30 mg/L and initial pH of supernatant = 6. After coagulation, a large amount of the fulvic-like acid and aromatic heterocyclic compounds were removed and the degree of complexity and aromaticity of organics decreased. After direct contact evaporation, using PAFC as coagulant still was the best selection due to its lowest concentrations of COD and NH3–N (22 mg/L and 1.02 mg/L) in the condensate produced by this two-stage treatment when initial pH of supernatant was 6 during evaporation and the condensate produced by this two-stage treatment met the water quality standard for using as supplying water for circulating cooling water system when temperature of hot air used for heating LC was at low temperature (250 °C). The fulvic-like acid and aromatic heterocyclic compounds in the condensate continuously reduced. Phenol, adamantane, 1-isocyanato, phthalic anhydrid, tri(2-chloroethyl) phosphat, Heptadecane, 2-methyl, ginsenol and Octadecane, 2-methyl- in the condensate obviously decreased. The effect of four coagulants as pretreatment on reducing the temperature of hot air used for evaporating LC was ranked as PAFC > PFS > PAC > PSAF. PSAF was not recommended due to the large amount of NH3–N produced when using PSAF to treat the LC.