High loaded moving bed biofilm reactors treating pulp & paper industry wastewater: Effect of hydraulic retention time, filling degree and nutrients availability on performance, biomass fractions and nutrients utilization

Abstract

For treating pulp and paper (P&P) industry wastewaters, the high-loaded/nutrient-limited moving bed biofilm reactor (MBBR) is frequently followed by an activated sludge, in the Biofilm-Activated Sludge (BAS) configuration. Evidences show that the MBBR performance relies on a complex surface-volume relation, affecting the biosolids dynamics. That subject was addressed in parallel lab-scale MBBRs, with carrier filling degrees of 15% and 45%, fed with P&P wastewater. The removal of chemical oxygen demand (COD) and utilization of nutrients were evaluated for varying hydraulic retention times (HRT, 1.6–4.9 h), and availabilities of nitrogen and phosphorous. Nutrients excess and 4.9 h HRT led to soluble COD removal close to 50% (totality of the biodegradable portion) in both reactors, but only 32% was achieved at 1.6 h HRT and 45% filling degree. Restrained wastewater-biosolids contact time rather than overload justified that, as the maximum capacity (Kincannon–Stover model, 30.6 kg sCOD/(m3 d)) was substantially higher than the apparent removal rates (≤ 14.1 kg sCOD/(m3 d)). The performance at 4.9 h HRT was matched at 3.2 h HRT with threefold filling ratio, which compensated the lower contact time. Higher HRT was also responsible for i) improving nutrients usage (up to 1.72 times higher sCOD/P and 1.47 sCOD/N); ii) superior suspended solids content, corresponding up to 30% of total biomass at 4.9 h, against 8.6% at 1.6 h; and iii) up to 2.45 times greater planktonic maximum specific activity. Nutrients restriction boosted the sCOD/nutrient consumption ratio up to 2.65 times for the limited nutrient and 1.70 for the abundant one, with minimal sCOD:N:P (100:0.70:0.14) at limited N and 4.9 h HRT.