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

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/(m(3) d)) was substantially higher than the apparent removal rates (<= 14.1 kg sCOD/(m(3) 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.

Automated summary

The study found that higher hydraulic retention times and appropriate filling ratios can enhance the performance of MBBR. Under nutrient-limited conditions, MBBR can more effectively remove COD and utilize nutrients.