Sludge Line Research Articles

Feasibility of biogas upgrading at a WWTP after pre-treatment application: Techno-economic assessment validation with pilot test data

Improving the efficiency of anaerobic digestion (AD) of sewage sludge (SS) is a critical step toward the achievement of energy neutrality in wastewater treatment plants (WWTPs), as required by the European Green Deal. This study used a comparative techno-economic assessment (TEA) to evaluate the feasibility of producing biomethane, at a WWTP, through upgrading biogas with a double-stage permeation membrane plant. The biogas was originally generated from the AD of a mixture of primary sludge (PS) and either raw or pre-treated waste activated sludge (WAS), where biological or thermo-alkali pre-treatments were applied to increase the WAS intrinsic low degradability.The TEA was supported by the results of pilot-scale tests, carried out on WAS, which mimicked (i) a traditional mesophilic AD process; (ii) a two-stage AD process, where a temperature-phased anaerobic digestion (TPAD, 3 days, 55 °C + 20 days, 38 °C) was performed to biologically pre-treat WAS; (iii) a traditional mesophilic AD process preceded by a thermo-alkali (4 g NaOH/100 g TS, 90 °C, 90 min) pre-treatment.The TEA was carried out in two phases. In the first, the minimum size of the WWTP capable of making the costs necessary for the implementation of the biogas upgrading plant equal to the revenues coming from selling biomethane (at 62 €/MWh) in 10 years was calculated in the absence of pre-treatments. It resulted of 500,000 equivalent inhabitants (e.i.). In the second phase, for the WWTP size found previously, the effect of either biological or thermo-alkali pre-treatments on the economic balance was evaluated, that is the gain (or the loss) associated to the selling of biomethane, compared to the reference price of 62 €/MWh. It was found that the TPAD increased the biogas productivity by only 23.6%, too little to compensate the amount of heat necessary for the pre-treatment and the purchase cost of the additional reactor. Conversely, the thermo-alkali pre-treatment, which enhanced the WAS biogas productivity by 110%, increased the biomethane revenues by approx. 10 €/MWh, compared to the scenario without pre-treatments. This study offers useful data to WWTP managers who want to introduce WAS pre-treatments, combined with interventions for biogas upgrading, in a new or existing sludge line of a WWTP.

Enrichment of nutrients from anaerobically digested centrate minimizing microplastics content using a combination of membrane processes

Centrifuge effluent, or centrate, is a liquid stream generated in the sludge line of wastewater treatment plants in the sludge dewatering process. Nutrients are solubilized in the anaerobic digestion of the sludge, mainly in the form of ammonium-nitrogen and phosphates. Thus, when the dewatering of the digested sludge is performed (usually by centrifugation), a sludge liquor stream enriched in nutrients is generated. However, it also contains microparticles, including microplastics, since most of the microparticles from the wastewater are transferred to the sludge treatment line in wastewater treatment plants. In this work, microplastics in centrate have been analyzed (counted and identified), and membrane technologies (ultrafiltration and forward osmosis) have been applied to concentrate nutrients and to obtain a microplastic-free stream for further nutrient recovery. Two alternative configurations have been compared, changing the application order of these processes. The results showed that obtaining a stream with a concentration higher than 6000 mg/L of ammonium has been possible by using ammonium sulfate (150 g/L) as a draw solution in the forward osmosis process. On the other hand, as a consequence of the ultrafiltration application, microparticles were concentrated in the reject stream up to 800 microparticles/L. At the same time, the permeate presents a lower concentration of microplastics without reducing the concentration of the nutrients. In this way, this pioneering study enables the production of a nutrient-enriched stream with reduced microplastic concentrations, that could be applied to the agricultural soil as biofertilizer.

Defining Best Available Technique Performances for Urban Wastewater Plants: A French Application

The treatment of wastewater (WW) and the resulting sludge in urban WW treatment plants (UWWTPs) is accompanied by a significant consumption of resources as well as the concomitant emission of pollutants into the environment. One of the objectives of the NEXT (next generation of wastewater treatment plants) project is to identify sufficiently long-term optimization opportunities for processes to be selected as best available techniques (BATs) within the meaning of the industrial emissions directive (IED). The methodological framework used in this article is based on multi-criteria statistical tools, processing data from industrial plants to classify the sites studied and identify the reference plants in order to propose BAT references and associated emission levels that are reachable. Two public databases available on the internet were used, and 1,010 plants served as samples. The application and results demonstrated the applicability of the methodology and validated the mathematical approach used, as the thresholds comply with current French and European regulations. In the framework of the analysis conducted, five techniques (membrane bioreactor, bacterial bed, sand filter, and high-load or medium-load activated sludge) were identified as BAT according to the range of regulatory classification requirements for WW treatment plants (WWTPs). In addition, an example of reference values that may constitute a possible basis for the regulatory thresholds has been proposed. In parallel, a few discussion points were identified, including the choice of the metric for the reference values, the lack of data for the sludge line limiting identification of the BATs to the water line alone, the approximate characterization in the databases of certain parameters, as well as the problem of technique coupling due to the identification of one main technique.

Nutrient Removal from Industrial Wastewater Systems Using Controlled Activated Sludge Treatment

Biological treatment with activated sludge as an alternative treatment was studied, and nitrogen and phosphorus were monitored. This scheme involves modifying the treatment tank by dividing it into anoxic and aerobic sections. According to this scheme, the process simulation was performed using Hydromantis' GPS-X 7.0 computer software. Wastewater discharges are usually generated by various industrial activities such as the milk (dairy) processing industry, petroleum processing refineries and slaughterhouses, which were studied in detail. The economic evaluation (cost estimation) of removing nutrients has been done using the CapdetWorks 4.0 computer simulation software. A cost sensitivity analysis has also been performed for the variable influent flows, altering the alpha factor for oxygen transfer during the aeration process and serving as a typical variable. The simulation procedure and economic evaluation have been carried out for a complete wastewater treatment plant, including both treatment lines (the water line and the sludge line simultaneously), and the derived results have been represented through illustrated graphs, tables, and meaningful diagrams.

Tracking the formation potential of vivianite within the treatment train of full-scale wastewater treatment plants

Phosphorus recovery is a vital element for the circular economy. Wastewater, especially sewage sludge, shows great potential for recovering phosphate in the form of vivianite. This work focuses on studying the iron, phosphorus, and sulfur interactions at full-scale wastewater treatment plants (Viikinmäki, Finland and Seine Aval, France) with the goal of identifying unit processes with a potential for vivianite formation. Concentrations of iron(III) and iron(II), phosphorus, and sulfur were used to evaluate the reduction of iron and the formation potential of vivianite. Mössbauer spectroscopy and X-ray diffraction (XRD) analysis were used to confirm the presence of vivianite in various locations on sludge lines. The results show that the vivianite formation potential increases as the molar Fe:P ratio increases, the anaerobic sludge retention time increases, and the sulfate concentration decreases. The digester is a prominent location for vivianite recovery, but not the only one. This work gives valuable insights into the dynamic interrelations of iron, phosphorus, and sulfur in full-scale conditions. These results will support the understanding of vivianite formation and pave the way for an alternative solution for vivianite recovery for example in plants that do not have an anaerobic digester.

Operational strategies enhancing sewage sludge minimization in a combined integrated fixed-film activated sludge – oxic settling anaerobic system

A lab-scale integrated fixed-film activated sludge (IFAS) reactor was mplemented with the oxic-settling anaerobic (OSA) cycle for reducing sewage sludge production through the addition of an anoxic/anaerobic sludge holding tank (SHT) along the sludge recycle line. The IFAS-OSA system was operated under the different hydraulic retention time (HRT) in the SHT (HRTSHT) of 12 h and 6 h, at an oxidation-reduction potential (ORP) < −91 mV and solid retention time (SRT) between 39 and 126 d. Furthermore, the effect of temperature increase in the SHT (TSHT) from ambient (19.8–25.6 °C) to mesophilic (35 °C) conditions was investigated. The system performances were monitored in terms of sludge minimization and dewaterability efficiencies as well as carbon and nutrients reduction. The observed sludge yield (Yobs) for the IFAS system was 0.37(±0.06) mg VSS/mg COD. After OSA implementation Yobs decreased by 32% and 46–65% at HRTSHT of 12 h and 6 h, respectively, indicating that prolonged exposure to anoxic/anaerobic conditions was not beneficial for sludge reduction. The lowest Yobs of 0.09(±0.05) mg VSS/mg COD (76% lower than that in the IFAS system) was obtained at an HRTSHT of 6 h and when TSHT was set at 35 °C. OSA implementation did not affect COD and NH4+ oxidation of the IFAS system (90–96% and 99%, respectively) and improved total nitrogen (TN) reduction (31–53%) due to improved denitrification in the SHT. On the contrary, sludge dewaterability worsened following OSA implementation, which was linked to the increased levels of exopolymeric substances in the suspended biomass.

Fate of glyphosate and its metabolite AminoMethylPhosponic acid (AMPA) from point source through wastewater sludge and advanced treatment

The fate of glyphosate and its metabolite AminoMethylPhosponic acid (AMPA) was followed at the catchment of the Sûre river, mainly characterized by small population density and small and medium-sized wastewater treatment plants (WWTPs). A high concentration of AMPA was found in water samples collected in inlet from different wastewater streams, the industry being the main contributor, while glyphosate resulted mainly in domestic origin. The two molecules were also monitored in the anaerobic digestion as in the supernatant produced after centrifugation (reject water). A total of 0.0713 and 2.24 g/d of glyphosate and AMPA respectively were regularly returned to the activated sludge tank (AST) indicating a 20% impact of the sludge management line on the global wastewater mass balance. Finally, the use of Constructed Wetlands (CWs) in Vertical Flow (VF) configuration was tested as a suitable technology to retain both glyphosate and AMPA (90 and up to 50% elimination respectively) and minimize their discharge into surface water.

Feasibility of a return-sludge nursery concept for mainstream anammox biostimulation: Creating optimal conditions for anammox to recover and grow in a parallel tank

To overcome limiting anammox activity under sewage treatment conditions, a return-sludge nursery concept is proposed. This concept involves blending sludge reject water treated with partial nitritation with mainstream effluent to increase the temperature, N levels, and electrical conductivity (EC) of the anammox nursery reactor, which sludge periodically passes through the return sludge line of the mainstream system. Various nursery frequencies were tested in two 2.5 L reactors, including 0.5–2 days of nursery treatment per 3.5–14 days of the total operation. Bioreactor experiments showed that nursery increased nitrogen removal rates during mainstream operation by 33–38%. The increased anammox activity can be partly (35–60%) explained by higher temperatures. Elevated EC, higher nitrogen concentrations, and a putative synergy and/or unknown factor were responsible for 15–16%, 12–14%, and 10–36%, respectively. A relatively stable microbial community was observed, dominated by a “Candidatus Brocadia” member. This new concept boosted activity and sludge growth, which may facilitate mainstream anammox implementations based on partial nitritation/anammox or partial nitrification/denitratation/anammox.

Tracing the fate of microplastic in wastewater treatment plant: A multi-stage analysis of treatment units and sludge

Microplastic (MP) pollution is ubiquitous in the environment presenting a global problem for both scientists and the general public. One of the major pathways of MPs entering the natural environment is through wastewater treatment plants (WWTPs). Once MPs reach the natural environment, they are posing threat to aquatic ecosystems and public health. The aim of this study is to investigate the concentration, morphology, and composition of MPs in different treatment units of a WWTP. Sampling included different points across WWTP in the water and sludge lines. Pre-treatment of the samples consists of advanced Fenton oxidation, and alkaline and enzymatic digestion followed by density separation. Once the particles were isolated, their morphology and size were studied using a stereoscopic and optical microscope followed by final confirmation with ATR-FTIR and micro-FTIR spectroscopy. Microplastic particle concentrations exhibit significant reductions as water undergoes treatment in the WWTP. For summer sampling, concentrations decreased from 351 MP/L (influent) to 35 MP/L (primary clarifier), 32 MP/L (biological reactor), and 13 MP/L (2.3 MP/L) (secondary clarifier). Similarly, winter sampling showed reductions from 403 MP/L (influent) to 159 MP/L (primary clarifier), 178 MP/L (biological reactor), and 26 MP/L (5.6 MP/L) (secondary clarifier). Removal efficiency of WWTP is high and exceeds 96%. The most abundant morphology is fibers followed by fragments and films. Polymers such as PE, synthetic cellulose, PP, PVC, PE-PP, PEEA, PA, acrylamide, and PES are widely detected in different units of WWTP. The number of MPs that are avoided from being emitted into the environment through direct water discharge was estimated to be 9.1 × 1012 MP/year. Removed MPs tend to accumulate in the sludge that is used for agricultural purposes although it should be managed as waste properly, avoiding the transition of MPs pollutants to terrestrial ecosystems adding to the number of MPs that will inevitably end up in receiving water bodies through direct WWTP effluent discharge that was set in 5.1 × 1010 MP/year in the studied WWTP.

Life-cycle assessment and techno-economic evaluation of the value chain in nutrient recovery from wastewater treatment plants for agricultural application

The recovery of nitrogen and phosphorus is important to promote circular economy in wastewater treatment plants (WWTPs). In this study, the life cycle assessment (LCA) and techno-economic assessment (TEA) of a novel pilot-scale plant aimed at recovering ammonium nitrate and struvite for subsequent application in agriculture was conducted. The nutrient recovery scheme was implemented in the sludge line of the WWTP and included (i) struvite crystallisation and (ii) ion-exchange process combined with gas permeable membrane contactor. The LCA showed that using a fertilizer solution containing the recovered nutrients was environmentally better in most of the impact categories evaluated. Ammonium nitrate was the most important environmental contributor when using the recovered fertilizer solution as a result of the high consumption of chemicals needed for its production. The TEA illustrated that the implementation of the nutrient recovery scheme in the WWTP featured a negative net present value (NPV), primarily attributed to the high consumption of chemicals (representing 30 % of the gross cost). However, the implementation of the nutrient recovery scheme in the WWTP could be economically favourable if the cost of ammonium nitrate and struvite increased to 0.68 and 0.58 €/kg, respectively. The results of this pilot-scale study highlight that nutrient recovery considering the whole value chain for fertilizer application can be an attractive full-scale alternative from a sustainability point of view.

Full-Scale Sewage Sludge Reduction Technologies: A Review with a Focus on Energy Consumption

In recent years, increasing attention has been paid to the problem of sewage sludge management and the relevant energy consumption, which represent the main cost items in wastewater treatment plants. Therefore, implementation of technologies that can reduce sludge production and ensure a positive impact on the energy of the entire sewage treatment plant has gained considerable importance in the scientific and technical community. The objective of this study was thus to screen full-scale sludge reduction technologies integrated into both the water line and the sludge line of a municipal sewage treatment plant with a sustainable impact on the overall balance of the plant. The results showed that, within the water line, ultrasound in the recirculation line of the activated sludge allowed for greater reductions in sludge production than the Cannibal and UTN systems, despite the higher energy consumption. CAMBITM, BioThelysTM, ExelysTM and TurboTec® enabled the greatest reductions in sludge production among the technologies integrated into the sludge line, and although they required a large amount of energy, this was partially offset by energy recovery in terms of additional biogas production.

A comparative life cycle assessment (LCA) analysis of innovative methods employing cutting-edge technology to improve sludge reduction directly in wastewater handling units

Sludge reduction technologies can be used as an alternative plan to dispose of excess sludge in wastewater treatment plants, but the sustainability of these strategies is a challenging task. In this study, the potential environmental implications of innovative technologies based on lab-scale results published in the literature were assessed from the life cycle assessment (LCA) view point. Accordingly, three different scenarios using various sludge reduction mechanisms were selected and analyzed. The scenarios include (1) an integrated oxic-setting-anaerobic and ultrasound process; (2) a sequencing batch reactor (SBR) with an anaerobic side-stream reactor to its return activated sludge line; and (3) the anaerobic-anoxic-oxic system in conjunction with an ozone reactor. LCA was conducted through the CML-IA (baseline 2000) method. A sensitivity analysis was also done in order to determine how changes in the inputs are affecting the overall sustainability of the system. The environmental impact evaluation of three scenarios indicated that the integrated SBR with an anaerobic side-stream reactor was the most efficient and sustainable alternative in environmental terms. Although the upgraded SBR system was not the best alternative in terms of sludge reduction (65 % versus 78 % for Scenario 1 and 70 % for Scenario 3), it showed the highest overall performance and also demonstrated the lowest environmental impact compared to other scenarios. Because the scenarios studied were cutting-edge and provided novel sludge reduction strategies based on lab-scale results published in the literature, a comprehensive economic analysis and scale-up considerations are recommended to effectively assist decision-makers in guiding the development of emerging technologies.

Odor emission assessment of different WWTPs with Extended Aeration Activated Sludge and Rotating Biological Contactor technologies in the province of Cordoba (Spain)

In this study, five urban WWTPs (Wastewater Treatment Plant) with different biological treatment (Extended Aeration Activated Sludge – EAAS; Rotating Biological Contactor – RBC), wastewater type (Urban; Industrial) and size, were jointly evaluated. The aim was twofold: (1) to analyze and compare their odor emissions, and (2) to identify the main causes of its generation from the relationships between physico-chemical, respirometric and olfactometric variables. The results showed that facilities with EAAS technology were more efficient than RBC, with elimination yields of organic matter higher than 90%. In olfactometric terms, sludge managements facilities (SMFs) were found to be the critical odor source in all WWTPs compared to the Inlet point (I) or Post primary treatment (PP), and for seasonal periods with ambient temperature higher than 25 °C. Moreover, the global odor emissions quantified in all SMFs revealed that facilities with EAAS (C-WWTP, V-WWTP and Z-WWTP) had a lower odor contribution (19,345, 14,800 and 11,029 ouE/s·m2, respectively) than for those with RBC technology (P-WWTP and NC-WWTP) which accounted for 19,747 ouE/s·m2 and 80,061 ouE/s·m2, respectively. In addition, chemometric analysis helped to find groupings and differences between the WWTPs considering the wastewater (71.27% of total variance explained) and sludge management (64.52% of total variance explained) lines independently. Finally, odor emissions were adequately predicted from the physico-chemical and respirometric variables in the wastewater (r2 = 0.8738) and sludge (r2 = 0.9373) lines, being pH, volatile acidity and temperature (wastewater line), and pH, moisture, temperature, SOUR (Specific Oxygen Uptake Rate) and OD20 (Cumulative Oxygen Demand at 20 h) (sludge line) the most influential variables.

Sewage Sludge Management at District Level: Reduction and Nutrients Recovery via Hydrothermal Carbonization

In this study, two scenarios of a municipal wastewater treatment plant (WWTP) are presented, which include the integration of the hydrothermal carbonization (HTC) process into the sludge line as a post-treatment of the anaerobic digestion (AD) process. The objective of the simulation is to investigate the performances of AD + HTC treatment to reduce sludge production and improve nutrient and energy recovery. For this purpose, the scheme of an under-construction WWTP was considered, named Trento 3 (Trento, Italy) and with a treatment capacity of 300,000 PE. In the first scenario, the HTC process was fed with thickened sludge from the Trento 3 WWTP, while in the second scenario, dewatered sludge from other local WWTPs was also used as feedstock for the HTC process. Both scenarios allowed to obtain a considerable sludge reduction ranging from 70 to 75% with a notably increase in the biogas production up to 47%, due to the recycling of HTC liquor (HTCL) to the anaerobic digester. Considering nutrients recovery, all the phosphorus and nitrogen present in the HTCL could be used for struvite precipitation with an average yearly gain of 1 million euros. Moreover, the introduction of HTC in the Trento 3 WWTP could allow a reduction in the sludge management costs of up to 2 M€/year.Graphical

Screening of microplastics in water and sludge lines of a drinking water treatment plant in Catalonia, Spain

Microplastics (MPs) are emerging pollutants detected everywhere in the environment, with the potential to harm living organisms. The present study investigated the concentration, morphology, and composition of MPs, between 20 μm and 5 mm, in a drinking water treatment plant (DWTP) located close to Barcelona (Catalonia, NE Spain). The sampling included different units of the DWTP, from influent to effluent as well as sludge line. Sampling strategy, filtration, allows sampling of large volumes of water avoiding sample contamination, and during 8 h in order to increase the representativeness of MPs collected. The pre-treatment of the samples consisted of advanced oxidation with Fenton's reagent and hydrogen peroxide, followed by density separation of the particles with zinc chloride solution. Visual identification was performed with an optical and stereoscopic microscope with final Fourier-transform infrared spectroscopic (FTIR) confirmation. MPs were found in all DWTP samples, with concentrations from 4.23 ± 1.26 MPs/L to 0.075 ± 0.019 MPs/L in the influent and effluent of the plant, respectively. The overall removal efficiency of the plant was 98.3%. The most dominant morphology was fibers followed by fragments and films. Twenty-two different polymer types were identified and synthetic cellulose, polyester, polyamide, polypropylene, polyethylene, polyurethane, and polyacrylonitrile were the most common. Although MPs could be incorporated from the distribution network, MPs intake from drinking water from this DWTP was not an important route compared to fish and seafood ingestion.

Improving wastewater treatment plant performance based on effluent quality, operational costs, and reliability using control strategies for water and sludge lines

As a result of urbanization and population growth, wastewater treatment plants (WWTPs) are experiencing increased influent flow rates, pollutant loads, and stricter standards for effluent quality. Therefore, implementing a control strategy is critical for improving the performance of WWTPs in response to highly variable effluent quality. In this study, different strategies for both water and sludge lines are deployed in WWTP model to control dissolved oxygen and biomass concentration in activated sludge system by manipulating aeration intensity and sludge flow rates. The efficiency of suggested control strategies is evaluated using different performance criteria, including effluent quality indicators, operational costs, and system reliability. The proposed method is applied in a real WWTP located in south of Tehran, the capital of Iran. According to the findings, controlling aeration system and sludge flow rate improve effluent quality and system reliability despite increasing operational costs. In the case of simultaneous control of water and sludge lines, the effluent quality index has improved by 9.3%, while the operational costs have increased by 1%. Moreover, the system reliability based on effluent ammonia, TSS, and COD concentration have increased by 33.4%, 9%, and 12.2%, respectively.

Enhanced sewage sludge treatment via parallel anaerobic digestion at the upper mesophilic level

Sewage mixed sludge (MS) digestion performance was ameliorated implementing the parallel digestion model for primary sludge (PS) and secondary sludge (SS) (waste activated sludge) as domestic sewage sludge fractions rich in oil and grease content at the upper mesophilic level (40 °C). Optimization of the organic loading rate (OLR) was conducted in parallel semi-continuous bench-scale digesters for PS, SS and MS. Comparatively evaluated performance and biosolid quality parameters were methane production rates, volatile solid (VS) reduction, oil and grease and nutrient content, dewaterability and electrical conductivity (EC). OLR optimization indicated different retention time needs for PS and SS stabilization and enabled 18% and 93% higher VS loading and reduction, respectively, compared to MS digestion. Inhibitory effect followed an ascending pattern as a result of OLR increase in each digestion line acting on the hydrolysis of proteinaceous matter and acetogenesis rather than methanogenesis. A high number of long chain fatty acids was detected in the raw sludges. The enhancing effect of the upper mesophilic temperature was significant in SS digestion with increased biodegradability, oil and grease removal and microbial growth compared to digestion at 35 °C. The parallel digestion system and upper mesophilic temperature proved a useful tool to enhance VS loading and reduction without worsening the stabilized biosolids’ dewaterability as a feasible model in the existing and prospective municipal wastewater treatment plants (WWTPs). The weakness of the MS digestion was diagnosed as the lower synthesis degree of biomass induced by the dilution of the substrate in PS by SS mixing which weakened the microbial tolerance to high OLR and inhibition. The output indicated the potential of parallel AD, importance of the optimization for OLR and temperature to advance the performance and flexibility of the sludge line practice in municipal WWTPs.

Thermophilic biological fluidized bed reactor in sludge line reduces greenhouse gas emissions in wastewater treatment system

Greenhouse gas (GHG) emissions represent one of the main drawbacks of wastewater (WW) treatment. However, results of a detailed estimation of the emissions can be a valid tool to define optimal solutions for minimizing impact of WW treatment system on the environment. Thermophilic biological fluidized bed reactor (TBFBR) has been recently proposed as an alternative solution for biological sludge minimization in wastewater treatment plant (WWTPs). In this work, 5 diverse scenarios of sludge line composition were studied and combined with 5 diverse sludge disposal options. GHG emissions in 25 combinations were fully investigated to define optimal sludge treatment and disposal option. Results suggested that TBFBR help to reduce net emitted GHGs with respect to scenario with conventional stabilization treatment in sludge line (anaerobic digestion) (−32.3 ± 3.55 %) thanks to (i) the reuse in water line of the aqueous residue of TBFBR as alternative carbon source, (ii) the significant minimization of sludge production, and (iii) the contained impact of gross GHG emissions due to the energy consumption of this process. The strong minimization of sludge also led to a decisive reduction in GHG emissions in the subsequent phases of transport, additional treatments, and final disposal making the choice of the disposal option indifferent on the overall GHG emission estimation. Moreover, the coupling of processes for the simultaneous and preventive maximization of energy recovery (TCH, and AnaD) before sludge minimization in TBFBR determined a limited reduction of GHG emission compared to scenario with TBFBR alone (−3.71 ± 1.47 %).

Enhancement of sewage sludge thickening and energy self-sufficiency with advanced process control tools in a full-scale wastewater treatment plant

On their path to becoming sustainable facilities, it is required that wastewater treatment plants reduce their energy demand, sludge production, and chemical consumption, as well as increase on-site power generation. This study describes the results obtained from upgrading the sludge line of a full-scale wastewater treatment plant over 6 years (2015–2021) using three advanced process control strategies. The advanced process control tools were designed with the aim of (i) enhancing primary and secondary sludge thickening, (ii) improving anaerobic digestion performance, and (iii) reducing chemical consumption in the sludge line. The results obtained show that the use of advanced process control tools allows for optimising sludge thickening (increasing solids content by 9.5%) and anaerobic digestion (increasing both the removal of volatile solids and specific methane yield by 10%, respectively), while reducing iron chloride and antifoam consumption (by 75% and 53%, respectively). With the strategies implemented, the plant increased its potential energy self-sufficiency from 43% to 51% and reduced de-watered sludge production by 11%. Furthermore, the upgrade required a low investment, with a return of capital expense (CAPEX) in 1.98 years, which presents a promising and affordable alternative for upgrading existing wastewater treatment plants.

Enhancing nutrient recovery by optimizing phosphorus stripping of bio-P sludge: Experimental analysis and modeling

Phosphorus (P) from bio-P sludge can precipitate uncontrolled in the sludge treatment line and decrease the potential for phosphorus recovery. The presence of an anaerobic unit (or equivalent) before the pre-dewatering stage could favor the early release of phosphorus and minimize uncontrolled struvite precipitation in the anaerobic digestion (AD) and post-digestion processes. In this study, a series of batch experiments in a phosphorus stripping unit (P- stripping unit) were conducted to investigate the phosphorus release rate and efficiency of phosphorus being stripped, when a bio-P sludge was exposed to different amounts of carbon and different temperatures (~20 °C and between 11 and 13 °C). The results indicated that with a dose between 10,000–17,000 mg sCOD (mg P mg TS−1)−1 at ~20 °C and between 15,000–25,000 mg sCOD (mg P mg TS−1)−1 at 11–13 °C, can be released 40–65% and 30–38% luxury phosphorus, respectively. Calculations showed that 80% percent of the soluble phosphorus present in the P-stripping unit (468 Kg d−1) was following the reject water of the pre-dewatering facility. A hybrid mathematical model was developed using the data obtained from the batch kinetic experiments to explain the phosphorus release kinetics in the stripping unit. A good fit between the experiment and model-predicted data demonstrates the ability of the model to predict the phosphorus release under varying initial conditions. Mass balance calculations on P and Nitrogen (N) for the sludge pre and post-dewatering side streams, showed that struvite precipitates 1.6 times more in the existing than in the modified configuration which includes a P-stripping unit. Implementing a P-stripping unit would minimize the precipitation in the AD, enhance P-recovery, and reduce the phosphorus content of EBPR biosolids. The information gained in this study can be used to generate and optimize phosphorus-rich streams early in the sludge line and thereby enhance phosphorus recovery.