Ozonation of Activated Sludge of Pulp and Paper Industry for Reduction in Quantity and Contamination

Abstract

Aerobic biological treatment with activated sludge process is predominantly applied for the treatment of wastewater in integrated pulp and paper mill around the world. 50-70% of the biodegradable organic material is mineralized with generation of additional biological cells, typically termed as excess sludge or waste activated sludge (WAS). Handling and disposal of WAS is confronted with several difficulties; regulatory stringency due to contamination of organochlorine compounds is the greatest hurdle. The objectives of the present research are to reduce the net disposable biomass and organochlorine compounds in the integrated pulp and paper industry. The research was carried out on the ozonation of filamentous biosludge of both laboratory reactor and integrated pulp and paper mill, and flocculating biosludge in the batch mode in the laboratory set-up. Ozonated biosludge was post treated in laboratory scale activated sludge process. In the initial phase of the research, biosludge was characterized for different parameters and sludge yield was determined in the laboratory reactors. The concentration of adsorbable organic halogens (AOX) and extractable organic halogens (EOX) compounds in dewatered biosludge from integrated pulp and paper mill was 2119 and 641 mg/kg respectively. Ten chlorophenolic compounds out of 12 compounds identified for regulation by USEPA were detected in the biosludge and the combined concentration was 2043 µg/kg. Toxicity equivalency (TEQ) due to chlorophenolic compounds in the biosludge was 717 µg/kg. The concentration of 2,3,7,8-TCDD and 2,3,7,8-TCDF was 16 and 210 pg/g of dry sludge respectively; the upper bound I-TEQ from PCDD/Fs was 54 pg/g DS. The concentration of organochlorine compounds in sludge was dependent on the concentration of the same in influent wastewater. The application of ozone on biosludge altered its physical, chemical and biological characteristics. With increase in ozone dosage and subsequent dissolution of biosludge, the CODs content in the aqueous phase was increased. The solubilised material was highly biodegradable. The BOD to CODs ratio which was 6.2 in the untreated wastewater increased to 69.6% at 92.3 mg O3/g DS dosage. Due to desorption of lignin from biosludge, increase in lignin and subsequent colour in the aqueous phase was observed. During ozonation of biomass, the AOX and EOX compounds were exposed for oxidation, which resulted in 41 and 67% removal of these compounds respectively at 92.3 mg O3/g DS dosage. The toxicity equivalency of total chlorophenolic compounds was decreased by 52.7% after ozone treatment; none of these compounds was detected in the aqueous phase after ozone treatment. Similarly, 92-100% removal of PCDD and PCDF compounds was observed at an ozone dosage of 46 mg/g DS. The upper bound I-TEQ from PCDD/Fs was reduced to 3.7 pg/g DS. The lysis of filamentous organisms and reduction in zeta potential made the biosludge settling in nature. The compact and dense ï¬ ocs were found resistant towards ozone attack, whereas the diffused and pinpoint ï¬ ocs were prone to ozonation. Ozonation of three times of excess sludge at an average ozone dosage of 46.4 mg/g DS followed by biological oxidation was capable to reduce significantly the disposal of waste activated sludge. The sludge yield in control and ozone bioreactors was 0.31 and 0.059 g/g of CODs removal. The recycling of ozonated biosludge to activated sludge process and thereby extra load of pollutants did not affect the performance of the process and removal efficiency of pollutants like COD, colour, AOX etc. There was no additional build-up of AOX, EOX and chlorophenolic compounds in the biosludge due to recycling of ozonated biosludge. The two stage process comprising of ozonation of biosludge coupled with biological treatment in activated sludge process resulted in 80.1, 81.2 and 79.9% lower discharge of AOX, EOX and chlorophenolic compounds with sludge respectively without affecting the concentration of these parameters in the treated wastewater. The process was effective for controlling the proliferation of filamentous organisms in the biological reactor which resulted in good settling characteristics of biosludge. The good viable count and better oxygen uptake rate in the bioreactor revealed that the activity of organism was better in the bioreactor. Ozonation can be a potential oxidative pretreatment process for reducing the WAS and paving the way for cost effective overall treatment of hazardous WAS in pulp and paper industry. Few recommendations have also been proposed based on the outcome of the research work.