Nitrogen removal in a cascade of four reactors employing the activated sludge process

The activated sludge process is the most widely used process for the biological treatment of domestic and industrial waste waters. Wastewater treatment plants based on the activated sludge process are in widespread use in developed and developing countries. The activated sludge model number 1 (ASM #1) is an internationally accepted standard for activated sludge modeling. It describes nitrogen and chemical oxygen demand within suspended-growth treatment processes, including mechanisms for nitrification and denitrification. We analyse the biological treatment of a wastewater when a cascade of four reactors is used. We assume that each reactor in the cascade has the same volume. Operating conditions are investigated in which the first reactor is not aerated whilst the last two reactors are aerated. The second reactor may either be aerated or not aerated. The process configuration includes one settling unit and one recycle unit. The settling unit is placed after the final reactor of the cascade. Part of its exit stream is wasted and the remainder is fed into the first reactor. The recycle unit is also placed after the final reactor of the cascade. The entirety of its exit stream is fed into the first reactor. The performance of a wastewater treatment plant can be characterised by a number of process parameters.Here we consider the nitrogen concentration in the effluent stream leaving the treatment plant (TNe). When the reactor configuration includesa settling unit this is defined by TNe=SNO+SNH+SND, where the state variables on the right hand are the concentration of soluble nitrate and nitrite nitrogen (SNO),soluble ammonium nitrogen (SNH), and soluble biodegradable organic (SND) respectively. A combination of direct numerical integration and continuation methods are used to investigate the steady-state behaviour of the system. The governing equations were solved using both matlab (ode15s) and maple(lsode[backfull]). For continuation XPPAUT was used. We take the hydraulic retention time (HRT) as the bifurcation parameter, primarily allowing it to vary over the range

Suitability of Membrane Bioreactor for treatment of recalcitrant textile dye wastewater utilising white-rot fungi

The performance of a laboratory scale membrane bioreactor (MBR) — utilizing a mixed microbial community dominated by fungi— for treatment of textile dye wastewater was investigated. A synthetic wastewater containing dye, starch (main contributor to total organic carbon, TOC) and other nutrients was used. Preliminary batch tests confirmed the superior decoloration capacity of pure fungus culture (Coriolus versicolor, NBRC 9791) as compared to that of conventional activated sludge. Simultaneous biosorption and biodegradation was evident in case of the fungus, while mainly biosorption was responsible for decoloration by activated sludge. On the other hand, activated sludge demonstrated comparatively faster TOC removal. Interestingly, stable removal of both color (over 99%) and TOC (over 98%) was achieved in the MBR under a hydraulic retention time (HRT) of 1 day. The difference of reactor-supernatant and membrane-permeate quality substantiated the significant contribution of the membrane to the overall dye removal (biosorption, cake layer filtration, biodegradation)

Removal of carbamazepine and sulfamethoxazole by MBR under anoxic and aerobic conditions

This study reveals for the first time that near-anoxic conditions (dissolved oxygen, DO - 0.5 mg/L) can be a favorable operating regime for the removal of the persistent micropollutant carbamazepine by MBR treatment. The removal efficiencies of carbamazepine and sulfamethoxazole by an MBR were systematically examined and compared under near-anoxic (DO\u27 0.5 mg/L) and aerobic (DO\u3e 2 mg/L) conditions. Preliminary batch tests confirmed that sulfamethoxazole is amenable to both aerobic and anoxic biotransformation. However, carbamazepine-a known persistent compound-showed degradation only under an anoxic environment. In good agreement with the batch tests, during near-anoxic operation, under a high loading of 750 J.l8 /Ld, an exceptionally high removal (68 ± 10%) of carbamazepine was achieved. In contrast, low removal efficiency (12 ± 11%) of carbamazepine was observed during operation under aerobic conditions. On the other hand, an average removal efficiency of 65% of sulfamethoxazole was achieved irrespective of the DO concentrations

Water extraction from mixed liquor of an aerobic bioreactor by forward osmosis: membrane fouling and biomass characteristics assessment

This study investigated membrane fouling and biomass characteristics during water extraction from mixed liquor of an aerobic bioreactor by a submerged forward osmosis (FO) system. As the sludge concentration in the reactor increased from 0 to 20 g/L, fouling of the FO membrane increased but was much less severe than that of a reference microfiltration membrane. The results also indicate that aeration can be used to effectively control membrane fouling. By increasing the draw solute concentration, as expected, the initial water flux was increased. However, there appears to be a critical water flux above which severe membrane fouling was encountered. A short-term osmotic membrane bioreactor experiment showed build-up of salinity in the bioreactor due to the reverse draw solute transport and inorganic salts rejection by the FO membrane. Salinity build-up in the bioreactor reduced the permeate flux and sludge production, and at the same time, altered the biomass characteristics, leading to more soluble microbial products and less extracellular polymeric substances in the microbial mass. Additionally, the inhibitory effects of the increased salinity on biomass and the high rejection capacity of FO led to the build-up of ammonia and ortho-phosphate in the bioreactor

Removal of trace organics by MBR treatment: the role of molecular properties

This study examined the relationship between specific molecular features of trace organic contaminants and their removal efficiencies by a laboratory scale membrane bioreactor (MBR). Removal efficiencies of 40 trace organic compounds were assessed under stable operating conditions. The reported results demonstrate an apparent correlation between chemical structures and the removal of trace organic contaminants by the laboratory scale MBR system. The removal of all 14 very hydrophobic (Log D \u3e 3.2) trace organic compounds selected in this study was consistently high and was above 85%. The occurrence and types of electron withdrawing or donating functional groups appear to be important factors governing their removal by MBR treatment. In this study, all hydrophilic and moderately hydrophobic (Log D \u3c 3.2) compounds possessing strong electron withdrawing functional groups showed removal efficiency of less than 20%. In contrast, high removal efficiencies were observed with most compounds bearing electron donating functional groups such as hydroxyl and primary amine groups. A qualitative framework for the assessment of trace organic removal by MBR treatment was proposed to provide further insights into the removal mechanisms

Effects of sludge retention time on oxic-settling-anoxic process performance: biosolids reduction and dewatering properties

In this study, the effect of sludge retention time (SRT) on oxic-settling-anoxic (OSA) process was determined using a sequencing batch reactor (SBR) attached to external aerobic/anoxic reactors. The SRT of the external reactors was varied from 10 to 40 d. Increasing SRT from 10 to 20 d enhanced volatile solids destruction in the external anoxic reactor as evidenced by the release of nutrients, however, increasing the SRT to 40 d did not enhance volatile solids destruction further. Relatively short SRT (10–20 d) favoured the conversion of destroyed solids into inert products. The application of an intermediate SRT (20 d) of the external reactor showed the highest sludge reduction performance (\u3e35%). Moreover, at the optimum SRT, OSA improved sludge dewaterability as demonstrated by lower capillary suction time and higher dewatered cake solids content

The role of forward osmosis and microfiltration in an integrated osmotic-microfiltration membrane bioreactor system

This study investigates the performance of an integrated osmotic and microfiltration membrane bioreactor (O/MF-MBR) system for wastewater treatment and reclamation. The O/MF-MBR system simultaneously used microfiltration (MF) and forward osmosis (FO) membranes to extract water from the mixed liquor of an aerobic bioreactor. The MF membrane facilitated the bleeding of dissolved inorganic salts and thus prevented the build-up of salinity in the bioreactor. As a result, sludge production and microbial activity were relatively stable over 60 days of operation. Compared to MF, the FO process produced a better permeate quality in terms of nutrients, total organic carbon, as well as hydrophilic and biologically persistent trace organic chemicals (TrOCs). The high rejection by the FO membrane also led to accumulation of hydrophilic and biologically persistent TrOCs in the bioreactor, consequently increasing their concentration in the MF permeate. On the other hand, hydrophobic and readily biodegradable TrOCs were minimally detected in both MF and FO permeates, with no clear difference in the removal efficiencies between two processes

Factors governing the pre-concentration of wastewater using forward osmosis for subsequent resource recovery

This study demonstrated a technique using forward osmosis (FO) to pre-concentrate the organic matter in raw wastewater, thereby transforming low strength wastewater into an anaerobically digestible solution. The chemical oxygen demand (COD) of raw wastewater was concentrated up to approximately eightfold at a water recovery of 90%. Thus, even low strength wastewater could be pre-concentrated by FO to the range suitable for biogas production via anaerobic treatment. Excessive salinity accumulation in pre-concentrated wastewater was successfully mitigated by adopting ionic organic draw solutes, namely, sodium acetate, and EDTA-2Na. These two draw solutes are also expected to benefit the digestibility of the pre-concentrated wastewater compared to the commonly used draw solute sodium chloride. Significant membrane fouling was observed when operating at 90% water recovery using raw wastewater. Nevertheless, membrane fouling was reversible and was effectively controlled by optimising the hydrodynamic conditions of the cross-flow FO system

Effects of thermal pre-treatment and recuperative thickening on the fate of trace organic contaminants during anaerobic digestion of sewage sludge

This study examined the effects of thermal pre-treatment and recuperative thickening on anaerobic digestion of sewage sludge on biogas production and removal of trace organic contaminants (TrOCs). Thermal pre-treatment and recuperative thickening resulted in approximately 15% increase in biogas production. However, the effects of thermal pretreatement and recuperative thickening on anaerobic digestion performance in respect to the removal of TrOCs were less obvious and varied widely depending on the molecular properties of each compound. Of the 40 TrOCs monitored in this study, 16 TrOCs were detected in all primary sludge samples. Removal from the aqueous phase was negligible for most of these 16 TrOCs. Caffeine and paracetamol were the only two TrOCs with a high removal from the aqueous phase. In comparison to the aqueous phase, TrOC removal from the solid phase was considerably higher. Through a mass balance calculation, it was shown that thermal pre-treatment or a combination of thermal pre-treatment and recuperative thickening could enhance the biodegradation of five persistent TrOCs, namely TCEP, verapamil, clozapine, triclosan, and triclocarban by 17-50%

Continuous transformation of chiral pharmaceuticals in enzymatic membrane bioreactors for advanced wastewater treatment

This study demonstrates continuous enantiomeric inversion and further biotransformation of chiral profens including ibuprofen, naproxen and ketoprofen by an enzymatic membrane bioreactor (EMBR) dosed with laccase. The EMBR showed non-enantioselective transformations, with high and consistent transformation of both (R)- and (S)-ibuprofen (93 ± 6%, n = 10), but lower removals of both enantiomers of naproxen (46 ± 16%, n = 10) and ketoprofen (48 ± 17%, n = 10). Enantiomeric analysis revealed a bidirectional but uneven inversion of the profens, for example 14% inversion of (R)- to (S)- compared to 4% from (S)- to (R)-naproxen. With redox-mediator addition, the enzymatic chiral inversion of both (R)- and (S)-profens remained unchanged, although the overall conversion became enantioselective; except for (S)-naproxen, the addition of redox mediator promoted the degradation of (R)-profens only