Improvement of dewatering characteristics by co-digestion of rice straw with sewage sludge

Abstract

A continuous mesophilic co-digestion of sewage sludge and softened rice straw was conducted and the dewatering characteristics of digested sludge were evaluated by a dewatering experiment using a belt press. The digestion was operated with solid retention time (SRT) of 25 days, and the feeding ratio of sludge to rice straw was 1:0.5 (total solids base). After 129 days of stable operation, the properties of digested sludge were analysed; then five kinds of cationic coagulants were tested to select the optimal coagulants for dewatering, and two coagulants were selected and used in the dewatering experiment because of lower doses and lower moisture of sludge cakes. Sludge property analysis showed that by the addition of rice straw, the fibrous materials in the digested sludge increased remarkably and the normalized capillary suction time (CST) decreased significantly, indicating that the dewatering properties was improved. The results of dewatering experiment showed that by the addition of rice straw, specific filtration rate of digested sludge increased by 81.2% and 174.6%, respectively; water content of dewatered sludge cakes decreased by 8.2% and 13.4%, respectively. The dewaterability of digested sludge was suggested to be improved due to rice straw addition.

Effects of Biomass Addition on Organic Composition of Supernatant in Sludge Digestion Process

“…Methane (CH 4 ) yields of each substrate are summarized in Table 1, CH 4 of Run 2 was calculated using the data after pH adjustment, and CH 4 yield of RS in Run 3 was calculated assuming that the degradation of sludge was the same as that in Run 1. Average CH 4 yields of raw sludge and RS were estimated as 0.31 and 0.26 L/g-TS, respectively, and the results were in accordance with the authors' previous study [8], suggesting that the digestion was conducted stably in the present study. CH 4 yield of each substrate was also summarized on COD Cr base ( Table 1).…”

“…It is suggested that due to the high C/N ratio of RS, the organic acids are generated as intermediates of carbohydrates degradation, and further accumulate and lead to the decrease of pH. On the other hand, the results of methane production, TS, VS removal rates of sludge digestion (Run 1) and the co-digestion (Run 3) were in accordance with the results in our previous study [8]. Table 2 shows the average concentrations of DOC, soluble COD Cr , carbohydrates, proteins, humic matter, acetate and propionate in the digested sludge of each reactor.…”

“…have been widely studied, and researches mainly focused on the digestion process, such as methane generation, RS degradation, microbial community's variation, etc [4][5][6]. In addition, mixing RS in sludge digestion process has also been reported to improve the dewaterability of digested sludge due to high contents of fibrous contents [7,8]. However, despite the advantages of co-digestion, there is limited research on the possible adverse effects which should be taken into consideration when AcoD is utilized for practical application.…”