Aerobic granular sludge technology has been extensively studied over the past 20 years and is regarded as the upcoming new standard for biological treatment of domestic and industrial wastewaters. ...Aerobic granules (AG) are dense, compact, self-immobilized microbial aggregates that allow better sludge-water separation and thereby higher biomass concentrations in the bioreactor than conventional activated sludge aggregates. This brings potential practical advantages in terms of investment cost, energy consumption and footprint. Yet, despite the relevant advances regarding the process of AG formation, instability of AG during long-term operation is still seen as a major barrier for a broad practical application of this technology. This paper presents an up-to-date review of the literature focusing on AG stability, aiming to contribute to the identification of key factors for promoting long-term stability of AG and to a better understanding of the underlying mechanisms. Operational conditions leading to AG disintegration are described, including high organic loads, particulate substrates in the influent, toxic feed components, aerobic feeding and too short famine periods. These operational and influent wastewater composition conditions were shown to influence the micro-environment of AG, consequently affecting their stability. Granule stability is generally favored by the presence of a dense core, with microbial growth throughout the AG depth being a crucial intrinsic factor determining its structural integrity. Accordingly, possible practical solutions to improve granule long-term stability are described, namely through the promotion of minimal substrate concentration gradients and control of microbial growth rates within AG, including anaerobic, plug-flow feeding and specific sludge removal strategies.
•AGS technology is viewed as the future standard in wastewater biological treatment.•AGS stability during long-term operation is a main challenge for broad application.•Granule structure depends on the operational conditions and wastewater composition.•Controlling the microbial growth at low rates is a key factor for stabilizing AGS.•Advances in operational strategies to enhance AGS stability are herein reviewed.
Application of fine-mesh sieves (<0.35 mm) as pretreatment for municipal biological wastewater treatment gives an opportunity to recover resources and increase sustainability of wastewater treatment ...processes. Sieves are traditionally used for single stage mechanical treatment (typical mesh of 0.35 mm) or in combination with an MBR (typical mesh >0.7 mm). When sieves with a mesh of 0.35 mm are used on raw sewage we observed that cellulose fibres mainly originating from toilet paper are removed efficiently from the influent with a high recovery and purity. The application of sieves as pretreatment for conventional activated sludge processes has been evaluated based on pilot plant research at three WWTPs in the Netherlands. With sieving applied to the dry weather flow only the overall energy usage of the WWTP including sludge treatment can be decreased by at least 40% with a payback time of 7 years.
► Pilot research with influent sieves was executed at the municipal WWTP Blaricum. ► Case studies based on the results have been performed. ► Calculations proved acceptable payback time and improved energy balance. ► Recovery of cellulose fibres was possible. ► Remaining items are biological sludge production and degradability of cellulose.
•Fullscale biodrying installation treating 150kton of sludge per year.•Generation of energy (9.3MW) from sewage sludge.•Recovery of ammonium sulphate solution 40% (w/w) (7.3kton/year) from process ...air.•Total wet sludge weight reduction by 73%.•80% Odour reduction due to a biobed.
A full-scale biodrying installation was treating 150kton (wet weight) of dewatered waste activated sludge per year. The waste was treated at thermophilic conditions (65–75°C) in a 2-step forced aeration process reducing the total wet sludge weight by 73%. The final product had a high caloric value (7700–10,400 (kJ/kg)), allowing a combustion for energy generation in external facilities. The resulting product met the European microbial and heavy metal quality standards needed for an application as organic fertilizer. The facility used <0.5MW of electricity and recovered 9.3MW from biologically produced heat, which was internally used for the heating of office buildings. Produced ammonia, originating from the microbial conversion of organic matter, was recovered from the ventilated air in an acid gas scrubber as an ammonium sulphate solution 40% (w/w) (7.3kton/year) and was sold as substitute for artificial fertilizers. The sustainability of this process is discussed relative to other sludge handling processes.
Anaerobic ammonium oxidation, nitrification and removal of COD was studied at ambient temperature (18 °C ± 3) in an anoxic/aerobic granular sludge reactor during 390 days. The reactor was operated in ...a sequencing fed batch mode and was fed with acetate and ammonium containing medium with a COD/N ratio of 0.5 g COD/gN. During influent addition, the medium was mixed with recycled effluent which contained nitrate in order to allow acetate oxidation and nitrate reduction by anammox bacteria. In the remainder of the operational cycle the reactor was aerated and controlled at a dissolved oxygen concentration of 1.5 mg O
2/l in order to establish simultaneous nitritation and Anammox. Fluorescent in-situ hybridization (FISH) revealed that the dominant Anammox bacterial population shifted toward Candidatus “Brocadia fulgida” which is known to be capable of organotrophic nitrate reduction. The reactor achieved stable volumetric removal rates of 900 g N
2–N/m
3/day and 600 g COD/m
3/day. During the total experimental period Anammox bacteria remained dominant and the sludge production was 5 fold lower than what was expected by heterotrophic growth suggesting that consumed acetate was not used by heterotrophs. These observations show that Anammox bacteria can effectively compete for COD at ambient temperatures and can remove effectively nitrate with a limited amount of acetate. This study indicates a potential successful route toward application of Anammox in granular sludge reactors on municipal wastewater with a limited amount of COD.
► acetate oxidizing capacity of Anammox at ambient temperature conditions and low COD:N ratios. ► nitritation/anammox granular sludge system as main stream wastewater treatment processes. ► low sludge production. ► competition between Anammox and general heterotrophs for acetate.
Iron is an important element for modern sewage treatment, inter alia to remove phosphorus from sewage. However, phosphorus recovery from iron phosphorus containing sewage sludge, without ...incineration, is not yet economical. We believe, increasing the knowledge about iron-phosphorus speciation in sewage sludge can help to identify new routes for phosphorus recovery. Surplus and digested sludge of two sewage treatment plants was investigated. The plants relied either solely on iron based phosphorus removal or on biological phosphorus removal supported by iron dosing. Mössbauer spectroscopy showed that vivianite and pyrite were the dominating iron compounds in the surplus and anaerobically digested sludge solids in both plants. Mössbauer spectroscopy and XRD suggested that vivianite bound phosphorus made up between 10 and 30% (in the plant relying mainly on biological removal) and between 40 and 50% of total phosphorus (in the plant that relies on iron based phosphorus removal). Furthermore, Mössbauer spectroscopy indicated that none of the samples contained a significant amount of Fe(III), even though aerated treatment stages existed and although besides Fe(II) also Fe(III) was dosed. We hypothesize that chemical/microbial Fe(III) reduction in the treatment lines is relatively quick and triggers vivianite formation. Once formed, vivianite may endure oxygenated treatment zones due to slow oxidation kinetics and due to oxygen diffusion limitations into sludge flocs. These results indicate that vivianite is the major iron phosphorus compound in sewage treatment plants with moderate iron dosing. We hypothesize that vivianite is dominating in most plants where iron is dosed for phosphorus removal which could offer new routes for phosphorus recovery.
•Fe chemistry and its correlation with P in two sewage treatment plants (STPs) was studied.•Surplus and digested sludge solids in the investigated STPs were dominated by Fe(II).•The Fe(II)P mineral vivianite was a major iron compound in all samples.•We hypothesize: Vivianite is a key compound in all STPs and offers new P recovery routes.
The anaerobic acetate (HAc) uptake stoichiometry of phosphorus-accumulating organisms (PAO) in enhanced biological phosphorus removal (EBPR) systems has been an extensive subject of study due to the ...highly variable reported stoichiometric values (e.g. anaerobic P-release/HAc-uptake ratios ranging from 0.01 up to 0.93 P-mol/C-mol). Often, such differences have been explained by the different applied operating conditions (e.g. pH) or occurrence of glycogen-accumulating organisms (GAO). The present study investigated the ability of biomass highly enriched with specific PAO clades (‘Candidatus Accumulibacter phosphatis’ Clade I and II, hereafter PAO I and PAO II) to adopt a GAO metabolism. Based on long-term experiments, when Poly-P is not stoichiometrically limiting for the anaerobic VFA uptake, PAO I performed the typical PAO metabolism (with a P/HAc ratio of 0.64 P-mol/C-mol); whereas PAO II performed a mixed PAO-GAO metabolism (showing a P/HAc ratio of 0.22 P-mol/C-mol). In short-term batch tests, both PAO I and II gradually shifted their metabolism to a GAO metabolism when the Poly-P content decreased, but the HAc-uptake rate of PAO I was 4 times lower than that of PAO II, indicating that PAO II has a strong competitive advantage over PAO I when Poly-P is stoichiometrically limiting the VFA uptake. Thus, metabolic flexibility of PAO clades as well as their intrinsic differences are additional factors leading to the controversial anaerobic stoichiometry and kinetic rates observed in previous studies. From a practical perspective, the dominant type of PAO prevailing in full-scale EBPR systems may affect the P-release processes for biological or combined biological and chemical P-removal and recovery and consequently the process performance.
•The anaerobic stoichiometry of PAO I and II is intrinsically different.•Both PAO I and II exhibit a GAO metabolism for HAc-uptake under Poly-P depletion.•At high Poly-P content PAO I has faster HAc-uptake rates than PAO II.•PAO II has faster HAc-uptake rates than PAO I at low Poly-P content.•Prevalence of specific PAO clades may influence process performance.
Municipal wastewater treatment plants emit methane. Since methane is a potent greenhouse gas that contributes to climate change, the abatement of the emission is necessary to achieve a more ...sustainable urban water management. This requires thorough knowledge of the amount of methane that is emitted from a plant, but also of the possible sources and sinks of methane on the plant. In this study, the methane emission from a full-scale municipal wastewater facility with sludge digestion was evaluated during one year. At this plant the contribution of methane emissions to the greenhouse gas footprint were slightly higher than the CO2 emissions related to direct and indirect fossil fuel consumption for energy requirements. By setting up mass balances over the different unit processes, it could be established that three quarters of the total methane emission originated from the anaerobic digestion of primary and secondary sludge. This amount exceeded the carbon dioxide emission that was avoided by utilizing the biogas. About 80% of the methane entering the activated sludge reactor was biologically oxidized. This knowledge led to the identification of possible measures for the abatement of the methane emission.
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► Methane emission from a municipal WWTP was quantified in total and per unit process. ► The anaerobic digestion related unit processes caused three quarters of the emission. ► Dissolved methane was sampled with a method based on salting-out of dissolved gases. ► Better handling of ventilation air from sludge facilities can decrease methane emission. ► Dissolved methane was aerobically oxidized in the activated sludge tank.
Emission of NO and N
2O from a full-scale two-reactor nitritation–anammox process was determined during a measurement campaign at the Dokhaven–Sluisjesdijk municipal WWTP (Rotterdam, NL). The NO and ...N
2O levels in the off-gas responded to the aeration cycles and the aeration rate of the nitritation reactor, and to the nitrite and dissolved oxygen concentration. Due to the strong fluctuations in the NO and N
2O levels in both the nitritation and the anammox reactor, only time-dependent measurements could yield a reliable estimate of the overall NO and N
2O emissions. The NO emission from the nitritation reactor was 0.2% of the nitrogen load and the N
2O emission was 1.7%. The NO emission from the anammox reactor was determined to be 0.003% of the nitrogen load and the N
2O emission was 0.6%. Emission of NO
2 could not be detected from the nitritation–anammox system. Denitrification by ammonia-oxidizing bacteria was considered to be the most probable cause of NO and N
2O emission from the nitritation reactor. Since anammox bacteria have not been shown to produce N
2O under physiological conditions, it is also suspected that ammonia-oxidizing bacteria contribute most to N
2O production in the anammox reactor. The source of NO production in the anammox reactor can be either anammox bacteria or denitrification by heterotrophs or ammonia-oxidizing bacteria. Based on the results and previous work, it seems that a low dissolved oxygen or a high nitrite concentration are the most likely cause of elevated NO and N
2O emission by ammonia-oxidizing bacteria. The emission was compared with measurements at other reject water technologies and with the main line of the Dokhaven–Sluisjesdijk WWTP. The N
2O emission levels in the reject water treatment seem to be in the same range as for the main stream of activated sludge processes. Preliminary measurements of the N
2O emission from a one-reactor nitritation–anammox system indicate that the emission is lower than in two-reactor systems.
Phosphate can be recovered as struvite or apatite in fluidised bed reactors. Urine has a much higher phosphate concentration than sludge reject water, allowing simpler (and less expensive) process ...for precipitation of phosphates. A stirred tank reactor with a special compartment for liquid solid separation was used to precipitate struvite from urine. Magnesium ammonium phosphate as well as potassium magnesium phosphate are two forms of struvite that were successfully precipitated. Liquid/solid separation was very effective, but the compaction of struvite was rather poor in the case of potassium struvite. Crystals did not form clusters and maintained the typical orthorhombic structure. Ammonium struvite had slightly lower effluent phosphate concentrations, but an average of 95% of influent phosphate was removed regardless of ammonium or potassium struvite precipitation. Fluid mechanics is believed to be important and should inform further work.
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