Carbon footprint analysis of wastewater treatment processes coupled with sludge in situ reduction.

The goal of this study was to assess the impacts or benefits of sludge in situ reduction (SIR) within wastewater treatment processes with relation to global warming potential in wastewater treatment plants, with a comprehensive consideration of wastewater and sludge treatment. The anaerobic side-stream reactor (ASSR) and the sludge process reduction activated sludge (SPRAS), two typical SIR technologies, were used to compare the carbon footprint analysis results with the conventional anaerobic - anoxic - oxic (AAO) process. Compared to the AAO, the ASSR with a typical sludge reduction efficiency (SRE) of 30 % increased greenhouse gas (GHG) emissions by 1.1 - 1.7 %, while the SPRAS with a SRE of 74 % reduced GHG emissions by 12.3 - 17.6 %. Electricity consumption (0.025 - 0.027 kg CO2-eq/m3), CO2 emissions (0.016 - 0.059 kg CO2-eq/m3), and N2O emissions (0.009 - 0.023 kg CO2-eq/m3) for the removal of secondary substrates released from sludge decay in the SIR processes were the major contributor to the increased GHG emissions from the wastewater treatment system. By lowering sludge production and the organic matter content in the sludge, the SIR processes significantly decreased the carbon footprints associated with sludge treatment and disposal. The threshold SREs of the ASSR for GHG reduction were 27.7 % and 34.6 % for the advanced dewatering - sanitary landfill and conventional dewatering - drying-incinerating routes, respectively. Overall, the SPRAS process could be considered as a cost-effective and sustainable low-carbon SIR technology for wastewater treatment.

Unraveling roles of the intermediate settler in a microaerobic hydrolysis sludge in situ reduction process.

The roles of the intermediate settler in the sludge process reduction activated sludge process (SPRAS), and the influences of its hydraulic retention time (HRTST) on pollutant removal and sludge reduction were investigated. Prolonging HRTST from 3.0 to 4.5 and 6.0 h resulted in sludge reduction efficiencies increased from 46.8% to 61.5% and 62.7%. The sludge accumulation in the intermediate settler formed an anaerobic zone but inhibited methane production, and the alternating microaerobic and anaerobic environment in the sludge process reduction (SPR) module increased the microbial diversity and enriched the hydrolytic and fermentative bacteria. Prolonging HRTST accelerated dissolved organic matter release and elevated the degradation of refractory fraction, and improved the sludge properties of the SPRAS. Metagenomic analysis showed that the SPR module enhanced the glycolysis pathway and decoupling metabolism for sludge reduction. The results revealed that the intermediate settler plays dual roles in solid-liquid separation and sludge reduction metabolism.

Comparison on biological nutrient removal and microbial community between full-scale anaerobic/anoxic/aerobic process and its upgrading processes.

A comparative study was conducted for the anaerobic/anoxic/aerobic (AAO) process and its two upgrading processes, five-stage Bardenpho and AAO coupling moving bed bioreactors (AAO + MBBR), using long-term operation data of six full-scale wastewater treatment plants. The three processes all had good COD and phosphorus removal performance. The reinforcing effects of carriers on nitrification were moderate at full-scale applications, while the Bardenpho was advantageous in nitrogen removal. The AAO + MBBR and Bardenpho processes both had higher microbial richness and diversity than the AAO. The AAO + MBBR favored bacteria to degrade complex organics (Ottowia and Mycobacterium) and to form biofilms (Novosphingobium), and preferentially enriched denitrifying phosphorus-accumulating bacteria (DPB) (norank_o__Run-SP154) with the highest anoxic to aerobic phosphorus uptake rates of 65.3 % - 83.9 %. The Bardenpho enriched bacteria tolerant to varied environments (Norank_f__Blastocatellaceae, norank_o__Saccharimonadales, and norank_o__SBR103), and was more suitable for the upgrading of the AAO because of its excellent pollutant removal performance and flexible operation mode.

Optimization of a compact on-site stormwater runoff treatment system: Process performance and reactor design.

Stormwater runoff has become a major anthropogenic urban pollution source that threatens water quality. In this study, coagulation-sedimentation, and ammonium ion exchange and regeneration (AIR) modules were coupled as a CAIR system to efficiently treat stormwater runoff. In the coagulation module, 99.3%, 91.7%, and 97.0% of turbidity, total phosphorus, and chemical oxygen demand could be removed at an optimized poly-aluminum ferric chloride dosage of 30 mg/L, and the continuous experiment confirmed that the full load mode was more suitable for its rapid start-up. In the AIR module, dynamic ammonium removal indicated that the breakthrough time decreased with the rising initial concentration and superficial velocity. The Modified Dose Response (MDR) model described the ammonium exchange behavior better than the Thomas and the Bohart-Adams models. Then, a design flow of the ion exchange reactor was constructed by correlating constants in the MDR model with engineering parameters, and the ion exchange reactor was designed for continuous operation of the CAIR system. The average concentrations of chemical oxygen demand, total phosphorus, ammonium nitrogen, and total nitrogen in the effluent of the CAIR system were 7.22 ± 2.26, 0.17 ± 0.05, 1.49 ± 0.01, and 1.62 ± 0.02 mg/L, respectively. The almost unchanged exchange capacity and physicochemical properties after the multicycle operation confirmed the durability of zeolite for ion exchange. Techno-economic analysis suggested that the CAIR system is practically promising for stormwater management with efficient pollutants removal, small footprint, and acceptable operating cost.

Simultaneous sludge minimization and membrane fouling mitigation in membrane bioreactors by using a microaerobic - Settling pretreatment module.

Membrane fouling is the major obstacle for membrane bioreactors operated at a long sludge retention time to reduce sludge production. In this study, a sludge process reduction (SPR) module, consisting of a microaerobic tank and a settler, was inserted before an anoxic/oxic MBR (AO-MBR) to achieve dual objectives of fouling alleviation and sludge reduction. Three SPR-MBRs were operated to investigate influences of sludge recirculation ratios from the SPR settler to the microaerobic tank on process performance. Compared to AO-MBR, the SPR-MBRs reduced sludge production by 43.1-56.4% by maintaining sludge retention times above 175 d, and decreased foulant layer resistance and pore clogging resistance. Inserting SPR reduced the accumulation of dissolved organic matters and extracellular polymeric substances, enlarged sludge flocs, and decreased sludge viscoelasticity. However, increasing RSPR stimulated outward diffusion of extracellular polymeric substances and increased sludge viscosity. SPR-MBRs achieved effective sludge reduction by enriching hydrolytic (Trichococcus and Aeromonas) and fermentative genera (Lactococcus, Paludibacter, Macellibacteroides, and Acinetobacter) in the SPR, and alleviated membrane fouling by prohibiting the growth of extracellular polymeric substance-secreting bacteria and enriching filamentous bacteria to enlarge particle size. The results revealed that the SPR-MBR maximized sludge reduction with a very long sludge retention time, and alleviated membrane fouling synchronously.

Performance and microbial community of MBBRs under three maintenance strategies for intermittent stormwater treatment.

Maintaining microbial activities is a critical problem for biological treatment processes of stormwater runoff because of its intermittent nature. In this study, the suitability of the moving bed biofilm reactor (MBBR) was assessed for stormwater treatment by long-term dry - rainy alternation operation. Three strategies to maintain microbial activities during the dry period, including keeping idle (MBBRI), introducing river water throughout the period (MBBRC), and ahead of a rainy day (MBBRM), were investigated. COD and NH4+-N removal efficiencies declined linearly from 94.2 % and 94.7 % to 51.7 % and 64.6 %, respectively, after the 61-day operation with microbial activity and biomass decreased. Introducing river water adversely affected the process performance as MBBRC presented the highest declining rates of COD and NH4+-N removal efficiencies. Most genera in MBBRs decayed and their microbial communities developed towards individualization, especially in MBBRM because of its highest environmental variability. Keeping idle slightly alleviated the performance decline and formed a more stable microbial community structure. However, significantly deteriorating performance in all MBBRs after the long-term operation indicated that MBBRs were unsuitable for treating stormwater independently of intermittent nature.

Mainstream nitrogen separation and side-stream removal to reduce discharge and footprint of wastewater treatment plants.

The activated sludge process is efficient for pollutant removal, but was criticized for its large upfront investment and land area requirements. Improving nitrogen removal to levels sufficient to reduce eutrophication is a challenge to conventional nitrification and denitrification, which is limited by process configuration (with nitrate recirculation) and environmental inhibition. To satisfy stringent discharge standards within a compact plant footprint, a sustainable strategy by moving nitrogen removal from mainstream to side-stream is designed by a cycle of ammonium exchange, regeneration and nitrogen removal (AERN), combined with biological and physiochemical technologies. Ammonium was rapidly captured by ion exchangers, then exchanged into regenerant, and converted to N2 by chlorination or Sharon-anaerobic ammonia oxidation in the side-stream. The AERN cycle can be combined with a high-rate anaerobic/aerobic process and chemical phosphorus removal to construct a HAERN process, or inserted between a coagulation-sedimentation tank and a membrane bioreactor to construct a CAERNM process. Two AERN-based systems both achieved efficient pollutants removal (especially for nitrogen removal of 86.8-93.7%) in long-term running, and didn't impair exchange capacity and properties of ion exchangers. Compared with the conventional anaerobic/anoxic/aerobic process, AERN-based processes reduce land occupancy, upfront investments, and treatment costs by 59.9-71.1%, 25.5-38.0% and 2.3-31.0%, respectively.

Inhibitory effects of Ca2+ on ammonium exchange by zeolite in the long-term exchange and NaClO-NaCl regeneration process.

The inhibitory effects of calcium ion (Ca2+) on ammonium (NH4+) exchange by zeolite were investigated in the long-term exchange and sodium hypochlorite - sodium chloride (NaClO-NaCl) regeneration process, and alleviation measure was developed and validated in this study. The batch experiments indicated that NH4+ removal efficiency, exchange kinetics and equilibrium isotherms were significantly dependent on the coexisting Ca2+. The exchange capacity decreased from 0.58 to 0.40 mg g-1 by increasing initial Ca2+ concentration from 0 to 100 mg L-1. The inhibitory effect of Ca2+ on NH4+ exchange efficiency was fitted to the competitive inhibition Monod model with half-saturation rate constant of 134.7 mg L-1. Ca2+ addition reduced the NH4+ removal rate and lengthened the exchange equilibrium time of zeolite. Periodic precipitation of Ca2+ in the form of calcium carbonate from the used regenerant maintained the removal efficiency of NH4+ commendably by alleviating inhibition effect of Ca2+ and extended the working life of zeolite. The major chemical compositions of natural and regenerated zeolite were basically unchanged. Compared to Bohart-Adams model and Thomas model, the Dose-Response model could predict the breakthrough curve well, and the fitted parameter further confirmed that NaClO-NaCl regeneration with periodic Ca2+ removal is an effective method to maintain efficient NH4+ from wastewater by zeolite.