Nutritional ingredients and fertilizer potential of the dewatering effluent after sludge wet air oxidation

Wet air oxidation (WAO) is an effective technology for sludge treatment, and the treated sludge was mechanically dewatered to generate sludge cake and filtrate effluent. The sludge cake can be used as a soil conditioner or sent to landfills, but the effluent is usually sent back to wastewater treatment plants with nutrient waste. In this study, we proposed to use the effluent to process water-soluble organic fertilizer and analyzed its characteristics systematically and its effect on seed germination. The results showed that humic acid, protein, and polysaccharide were decomposed in different degrees in WAO with a chemical oxygen demand (COD) removal rate of 60%–70%. After a WAO treatment at 240°C, phosphorus, potassium, and heavy metals remained in the solid phase. At the same time, the effluent contained ammonium of more than 2800 mg/L, amino acids of nearly 3000 mg/L, humic acid derivatives of 2400 mg/L, and acetic acid of 3900 mg/L. Acetic acid was helpful in seed germination, while the other components were critical to root growth. The diluted effluent significantly prompted the germination and development of three kinds of plant seeds, particularly Spring Onion, with a germination index exceeding 200%.