Effect Of Sludge Retention Time Research Articles

Anaerobic stabilisation and conversion of biopolymers in primary sludge—effect of temperature and sludge retention time

The effect of sludge retention time (SRT) and process temperature on the hydrolysis, acidification and methanogenesis of primary sludge was investigated in completely stirred tank reactors (CSTRs). The CSTRs were operated to maintain SRTs of 10, 15, 20 and 30 days at process temperatures of 25°C and 35°C. The rates of hydrolysis and the biodegradability of primary sludge were assessed in batch reactors incubated at 15°C, 25°C and 35°C. The results revealed that the major amount of sludge stabilisation occurred between 0 and 10 days at 35°C and 10 and 15 days at 25°C. Hydrolysis was found to be the rate limiting-step of the overall digestion process, for the reactors operated at 35°C and 25°C, except for the reactor operated at 10 days and 25°C. At the latter conditions, methanogenesis was the rate-limiting step of the overall digestion process. Proteins hydrolysis was limited to a maximum value of 39% at 30 days and 35°C due to proteins availability in the form of biomass. The biodegradability of primary sludge was around 60%, and showed no temperature dependency. The hydrolysis of the main biopolymers and overall particulate COD of the primary sludge digested in CSTRs were well described by first-order kinetics, in case hydrolysis was the rate-limiting step. Similarly, the hydrolysis of the overall particulate COD of the primary sludge digested in batch reactors were described by first-order kinetics and revealed strong temperature dependency, which follows Arrhenius equation.

Effect of sludge retention time on microbial behaviour in a submerged membrane bioreactor

To clarify the effect of sludge retention time (SRT) on performance and microbial behaviour in a submerged membrane bioreactor for the treatment of domestic wastewater, four runs of a laboratory scale reactor with hydraulic retention time of 5 h and SRTs of 5, 10, 20 and 40 days, respectively, were conducted. The membrane bioreactor process was capable of achieving over 90% removals both for chemical oxygen demand and ammonia nitrogen, on average, almost independent of SRT. With prolonged SRT, concentrations of suspended solids and volatile suspended solids in the bioreactor increased accordingly, whereas sludge growth kinetic parameters, sludge yield and endogenous decay coefficients, declined slightly and exponentially, respectively. The mean sludge particle sizes at different SRTs were in the range 14.82–48.24 μm, providing a favourable environment for enhancement of mass transport. Characterized by an oxygen consumption rate, sludge specific activities both for organic decomposition and nitrification of ammonia nitrogen varied with SRT. However, volumetric oxygen consumption rates of the membrane bioreactor, representing the whole ability of the process for decomposing pollutants, were enhanced as SRT increased.

Effect of sludge retention time (SRT) on nutrient removal in sequencing batch reactors

Two bench‐scale activated sludge Sequencing Batch Reactors (SBR's), with working volumes of 100 liters each, operated on synthetic wastewater at sludge retention times (SRT) of 7 days (Reactor 1) and 15 days (Reactor 2) respectively. Biological phosphorus and nitrogen removals were applied in the systems to study nitrification, denitrification, and phosphorus removal kinetics during the start‐up phase. Excellent enhanced biological phosphorus removal was achieved in both reactors within 21 days. The maximum phosphorus release rate in each reactor was 10‐mg PO4‐P/ g.MLSS.h at 20°C and phosphorus contents of about 5% dry weight of biomass were achieved. In Reactor 2, very good nitrification was achieved with 100% ammonia removal within initial two weeks of operation. The maximum nitrification rate was 3.0 mg NO3‐N/g. MLSS.h, but later phosphorus release rates continued to decline, due to the elevated remaining nitrate concentrations from preceding cycles in the initial fill and mix phase. Remaining nitrate consumes influent organic during the fill and mix phase, decreasing the availability of organic matter for phosphorus removing bacteria, thus deteriorating their activity. For SBR 1, phosphorus release rates didnl shows any decline, as incomplete nitrification led to much lower nitrate during the initial fill and mix phase. The present study implied that higher SRT is beneficial for phosphorus removal. But later, SRT should be reduced to decrease the nitrification activity, if the target is phosphorus removal, otherwise remaining elevated nitrate concentration diminishes phosphorus removal.