Continuous-flow Activated Sludge Systems Research Articles

Investigation of Triclosan Fate and Performance in Continuous-Flow Activated Sludge Systems

Investigation of Triclosan Fate and Performance in Continuous-Flow Activated Sludge Systems

Application of aerobic kenaf granules for biological nutrient removal in a full-scale continuous flow activated sludge system

Aerobic granular sludge (AGS) is a biofilm technology that offers more treatment capacity in comparison to activated sludge. The integration of AGS into existing continuous-flow activated sludge systems is of great interest as process intensification can be achieved without the use of plastic-based biofilm carriers. Such integration should allow good separation of granules/flocs and ideally with minor retrofitting, making it an ongoing challenge. This study utilized an all-organic media carrier made of porous kenaf plant stalks with high surface areas to facilitate biofilm attachment and granule development. A 5-stage Bardenpho plant was upgraded with the addition of kenaf media and a rotary drum screen to retain the larger particles from the secondary clarifier underflow whereas flocs were selectively wasted. Startup took 5 months with a sludge volume index (SVI) reduction from >200 to 50 mL g−1. Most of the kenaf granules fell in the size range of 600–1400 μm and had a clear biofilm layer. The wet biomass density, SVI30, and SVI30/SVI5 of the kenaf granules were 1035 g L−1, 30.6 mL g−1, and 1.0, respectively, which met the standards of aerobic granules. Improved stability of biological phosphorus removal performance enabled a 25% reduction in sodium aluminate usage. Microbial activities of kenaf granules were compared with aerobic granules, showing comparable N and P removal rates and presence of ammonium-oxidizing bacteria and polyphosphate-accumulating organisms in the outer 50–60 μm layer of the granule. This work is the first viable example for integrating fully organic biofilm particles in existing continuous-flow systems.

Effect of Using Anaerobic Selectors on the Performance of Continuous Flow Activated Sludge Systems

Effect of Using Anaerobic Selectors on the Performance of Continuous Flow Activated Sludge Systems

Removal of selected endocrine disrupters in activated sludge systems: Effect of sludge retention time on their sorption and biodegradation

The fate of three endocrine-disrupting compounds (EDCs), namely 4- n-nonylphenol (4- n-NP), triclosan (TCS) and bisphenol A (BPA), was studied in activated sludge process. The effect of sludge retention time (SRT) on their sorption and biodegradation was also investigated. Three continuous-flow activated sludge systems were used, operating at SRT of 3, 10 and 20 days, respectively. Batch experiments with biomass originated from the aforementioned systems showed that 4- n-NP and TCS are significantly sorbed on the suspended solids, while BPA to a lesser extent. The sorption affinity of target compounds did not seem to be affected by SRT, with the exception of 4- n-NP for which higher values of sorption coefficients were calculated at SRT of 3 days. The operation of continuous-flow systems in the presence of 10–40 μg L −1 of EDCs indicated that more than 90%, 94% and 97% of BPA, TCS and 4- n-NP, respectively, can be removed during the activated sludge process in systems operating at SRT of 3–20 days. Determination of EDCs in the dissolved and particulate phase and calculation of their mass flux revealed that biodegradation was the principal removal mechanism of EDCs. For SRT values used in this study, this parameter does not seem to affect significantly percentage biodegradation of BPA. On the other hand, biodegradation of 4- n-NP and TCS was enhanced at SRT of 20 days. Calculation of biotransformation rates showed that the highest values for all EDCs were calculated at SRT of 3 days.

Loss of degradation capacity of activated sludge for a xenobiotic after a period without its influent

Xenobiotic shock experiments were conducted on lab-scale continuous-flow activated sludge systems to examine activated sludge treatment performance and to determine the xenobiotic degrader loss after periods of xenobiotic absence. The systems were operated with normal influent of a xenobiotic and a biogenic substrate until steady state, and were then artificially disturbed by removing and re-adding the xenobiotic in the influent. Substantial xenobiotic leaks were found when xenobiotic absent time was approximately one mean cell residence time ( θ c), and the system failed when xenobiotic absent time was longer than a θ c. Amount of degrader at the time of dual substrate steady state was estimated to be approximately 6% of the total sludge. As the xenobiotic absence time was lengthened, degrader amount in the system was reduced exponentially at a half life of approximately three days. The loss rate could be attributed mainly to the rate of displacement by θ c operation, followed by endogenous decay and de-acclimation loss.

Investigation of triclosan fate and toxicity in continuous-flow activated sludge systems

The purpose of this research was to study the fate and toxicity of triclosan (TCS) in activated sludge systems and to investigate the role of biodegradation and sorption on its removal. Two continuous-flow activated sludge systems were used; one system was used as a control, while the other received TCS concentrations equal to 0.5 and 2mgl(-1). At the end of the experiment, 1mgl(-1) TCS was added in the control system to investigate TCS behaviour and effects on non-acclimatized biomass. For all concentrations tested, more than 90% of the added TCS was removed during the activated sludge process. Determination of TCS in the dissolved and particulate phase and calculation of its mass flux revealed that TCS was mainly biodegraded. Activated sludge ability to biodegrade TCS depended on biomass acclimatization and resulted in a mean biodegradation of 97%. Experiments with batch and continuous-flow systems revealed that TCS is rapidly sorbed on the suspended solids and afterwards, direct biodegradation of sorbed TCS is performed. Regarding TCS effects on activated sludge process, addition of 0.5mgl(-1) TCS on non-acclimatized biomass initially deteriorated ammonia removal and nitrification capacity. After acclimatization of biomass, nitrification was fully recovered and further increase of TCS to 2mgl(-1) did not affect the performance of activated sludge system. The effect of TCS on organic substrate removal was minor for concentrations up to 2mgl(-1), indicating that heterotrophic microorganisms are less sensitive to TCS than nitrifiers.

Activated sludge treatment of a xenobiotic with or without a biogenic substrate during start-up and shocks

Lab-scale continuous flow activated sludge systems that were acclimated to 2,4-dichlorphenoxyacetic acid (2,4-D) under sole 2,4-D influent and without sludge wastage, were able to maintain successful 2,4-D treatment when both 2,4-D and a biogenic substrate were fed and the systems operated with finite mean cell residence times ( θ c). When the systems were fed dual 2,4-D and biogenic substrates and operated with finite θ c from the start, treatment of 2,4-D fluctuated noticeably long after acclimation. At the reintroduction of 2,4-D after its absence from the influent for a period of time (2,4-D shock), the systems under both the sole and dual substrate conditions suffered similar treatment losses; the extent of treatment losses was related to the length of 2,4-D absence time. When shocked, systems with sole 2,4-D influent had a slight advantage over dual substrates by showing a faster recovery from shocks with the help of re-acclimation.

Investigation of Cr(VI) reduction in continuous-flow activated sludge systems

The aim of this research was to investigate hexavalent chromium, Cr(VI), reduction by activated sludge and to evaluate the use of continuous-flow activated sludge systems for the treatment of Cr(VI)-containing wastewater. Three series of experiments were conducted using two parallel lab-scale activated sludge systems. During the first experiment, one system was used as a control, while the other received Cr(VI) concentrations equal to 0.5, 1, 3 and 5 mg l −1. For all concentrations added, approximately 40% of the added Cr(VI) was removed during the activated sludge process. Determination of chromium species in the dissolved and particulate phase revealed that the removed Cr(VI) was sorbed by the activated sludge flocs mainly as trivalent chromium, Cr(III), while the residual chromium in the dissolved phase was mainly detected as Cr(VI). Activated sludge ability to reduce Cr(VI) was independent of the acclimatization of biomass to Cr(VI) and it was not affected by the toxic effect of Cr(VI) on autotrophic and heterotrophic microorganisms. During the second experiment, both systems were operated under two different hydraulic residence time ( θ equal to 20 and 28 h) and three different initial organic substrate concentration (COD equal to 300, 150 and 0 mg l −1). Cr(VI) reduction was favored by an increase of θ, while it was limited by influent COD concentration. Finally, at the last experiment the effect of anoxic and anaerobic reactors on Cr(VI) reduction was investigated. It was observed that the use of an anoxic zone or an anaerobic–anoxic zone ahead of the aerobic reactor favored Cr(VI) reduction, increasing mean percentage Cr(VI) reduction to almost 80%.

Influence of the enhanced biological phosphorus removal on the waste activated sludge production

The effect of the enhanced phosphorus removal process (EBPR) on the waste activated sludge production and the type of phosphorus storage were investigated in two continuous flow activated sludge systems on semi-technical scale. One of the plants (plant II) was operated with the A/O process, whereas the other plant (plant I) was operated in a conventional, fully aerobic mode and served as a control. By monitoring the elementary composition of the activated sludge solids in plant II it was proved that a major part of the enhanced phosphorus removal was due to a storage as polyphosphate (poly-P). The additional uptake of phosphorus resulted in an increase of the inorganic sludge mass which was determined as 3.05 g SS/g P using the results of the measurements of the non volatile solid fraction. This value was also experimentally confirmed by the measurement of the difference between the WAS production of plant I and plant II. Based on the specific WAS production, an additional dry solids production of 3.14 g SS/g P was calculated. No indications for a significant difference of the organic sludge production between both plants were found, although the organic WAS production was slightly higher in experimental periods with a relatively high phosphorus content of the activated sludge solids of plant II.

Influence of the enhanced biological phosphorus removal on the waste activated sludge production

The effect of the enhanced phosphorus removal process (EBPR) on the waste activated sludge production and the type of phosphorus storage were investigated in two continuous flow activated sludge systems on semi-technical scale. One of the plants (plant II) was operated with the A/O process, whereas the other plant (plant I) was operated in a conventional, fully aerobic mode and served as a control. By monitoring the elementary composition of the activated sludge solids in plant II it was proved that a major part of the enhanced phosphorus removal was due to a storage as polyphosphate (poly-P). The additional uptake of phosphorus resulted in an increase of the inorganic sludge mass which was determined as 3.05 g SS/g P using the results of the measurements of the non volatile solid fraction. This value was also experimentally confirmed by the measurement of the difference between the WAS production of plant I and plant Il. Based on the specific WAS production, an additional dry solids production of 3.14 g SS/g P was calculated. No indications for a significant difference of the organic sludge production between both plants were found, although the organic WAS production was slightly higher in experimental periods with a relatively high phosphorus content of the activated sludge solids of plant II