Secondary Sludge Research Articles

Anaerobic co-digestion of sewage sludge and Cladophora green algae: Investigation of synergistic effects and Optimization of substrate composition ratio

Anaerobic co-digestion of primary and secondary sewage sludge and <i>Cladophora</i> green algae was investigated under mesophilic temperature conditions. The design of experiment method and the optimal mixture design were used to systematically optimize the substrate composition ratios and elucidate the possible synergistic effects for an anaerobic digestion system. A reduced cubic model was created by Design-Expert software as a function of substrate composition ratios. The model was validated by ANOVA experimentally. Also, the substrate composition ratio's effects on variations in biogas production were studied. All linear impacts on changes in biogas production were significantly observed, and interactions between substrates in combined digestion had synergistic impacts on biogas production rate. The highest amount of biogas (235.17-296.03 ml/g VS) was obtained with ratios of equal to 70-60%, B equal to 0-17%, and C equal to 18-30%. Eventually, the model optimization was performed to predict the optimal conditions to achieve the maximum biogas production rate. It was seen that the predicted and actual values of produced biogas in favorable conditions with an error of about 1.3% are well consistent. The authors conclude that the optimal mixture design can be utilized to assess the optimal composition of substrates in an anaerobic digestion system.

Comparison of ultrasonic treatment of primary and secondary sludges: Physical properties and chemical properties

Excess sludges were by-product of sewage treatment and ultrasonic treatment was an efficient pretreatment of excess sludge. However, the difference of primary and secondary sludges treated by ultrasound unclear. This paper systematically studied the ultrasonic treatment of primary and secondary sludges, including changes of sludge particle size, sludge sedimentation, release of carbon, nitrogen, and phosphorous. These results showed that COD, TN, NH4+-N and TP in supernatant increased by 126.9 %, 76.4 %, 60.8 % and 38.5 %, respectively after ultrasonic treatment of primary sludge; and the secondary sludge was increased by 429.6 %, 66.5 %, 23.8 % and 13.7 %, respectively. After ultrasonic treatment, COD of secondary sludge increased more, and the C/N and C/P were higher. Besides, the sludge particle size was reduced from 48.42 to 9.40 μm for primary sludge, and from 41.20 to 12.46 μm for secondary sludge, respectively. That indicated ultrasound could crack sludge effectively, and the effect of secondary sludge lysis was better. Secondary sludge released more biodegradable substances with ultrasonic treatment. In addition, after ultrasonic treatment, the chemical structure of sludge changed little, but the chemical properties changed strongly, especially for the secondary sludge. That revealed ultrasound had little effect on the chemical structure of primary and secondary sludges. This paper will provide guidance in study of primary and secondary sludge with ultrasonic treatment.

An Assessment of Mass Flows, Removal and Environmental Emissions of Bisphenols in a Sequencing Batch Reactor Wastewater Treatment Plant

This study analyzed 16 bisphenols (BPs) in wastewater and sludge samples collected from different stages at a municipal wastewater treatment plant based on sequencing batch reactor technology. It also describes developing an analytical method for determining BPs in the solid phase of activated sludge based on solid-phase extraction and gas chromatography-mass spectrometry. Obtained concentrations are converted into mass flows, and the biodegradation of BPs and adsorption to primary and secondary sludge are determined. Ten of the sixteen BPs were present in the influent with concentrations up to 434 ng L-1 (BPS). Only five BPs with concentrations up to 79 ng L-1 (BPA) were determined in the plant effluent, accounting for 8 % of the total BPs determined in the influent. Eleven per cent of the total BPs were adsorbed on primary and secondary sludge. Overall, BPs biodegradation efficiency was 81%. The highest daily emissions via effluent release (1.48 g day-1) and sludge disposal (4.63 g day-1) were for BPA, while total emissions reached 2 g day-1 via effluent and 6 g day-1 via sludge disposal. The data show that the concentrations of BPs in sludge are not negligible, and their environmental emissions should be monitored and further studied.

Assessment and accumulation of microplastics in sewage sludge at wastewater treatment plants located in Cádiz, Spain

Sludge from Wastewater treatment plants (WWTPs) have been determined as a sink of microplastics (MP) removed from wastewater. The aim of this research work has been to evaluate the presence of these pollutants in the sludge of seven WWTPs (five urban and two industrial), located in southern Spain. Samples were collected in the primary, secondary and digested sludge matrixes, MPs were extracted following wet peroxide oxidation and the removal of cellulose, finally the samples were analyzed according to their abundance, size (from 100 μm to 5 mm), shape, colour, and polymer type. Subsequently, the data obtained on the WWTPs were compared, the main difference among the WWTPs and different sample points showed high heterogeneity in terms of abundance of microplastics, due to the differences in the sludge loaded, the processes and the type of sludge. The results from this study established that the most abundant shape was fibers; regarding the size, 100–355 μm fraction was the most abundant, showing that the amount of MPs increased when the size decreased. Regarding the type of polymers, 23 were identified by ATR-FTIR. Further, Acrylate, PE, EAA and PP were the most abundant found polymers. The presence of MPs in the digested sludge varied from 0.02 ± 0.006 MP g DW−1 to 57.18 ± 20.69 MP g DW−1 in the WWTP 6 (food industry) and WWTP 3 (urban city over 212,000 inhabitants), respectively; higher abundance of MPs found in the primary sludge in respect to secondary sludge; in concordance with the removal from wastewater line reported in other studies. The results obtained showed that MPs were widely present in sludge, becoming a sink of these pollutants, estimating that among 8.05 · 104 and 1.77 · 109 MPs · day−1 were loaded to sludge; therefore, these facilities act as a significant source of MPs into agriculture when sludge is used as soil amendment.

Enhancement of hydrolysis efficiency and biogas production by treatment of secondary sludge with bacteriophage lysozymes

Anaerobic digestion of secondary sludge from wastewater treatment plants can degrade sludge while producing methane that can be burned as bioenergy. Hydrolysis of secondary sludge is the first step in anaerobic digestion, but is the rate-limiting step. In this study, to speed hydrolysis of secondary sludge, lysozymes from bacteriophage species T4, T7 and λ were applied to batch tests. The volatile suspended solids were removed by lysozymes at efficiencies of 23.6–50.1 %. Use of the lysozymes increased the reaction coefficient of biogasification of volatile suspended solids by 0.4–1.0 d−1, regardless of the origin of sludge, or sources of lysozyme. The removal efficiency of volatile suspended solids and methane yield had significantly positive correlation (p < 0.001). Relative abundances of genera Simplicispira (10.3 → 5.2 %), Dokdonella (4.9 → 3.9 %), and Thermomonas (2.7 → 1.2 %) were high in original secondary sludge, and maintained relative abundance over other genera even after lysozyme treatment. To increase the ratio of microbes to organics, sludge samples from various sources were aerated and cultivated with 40 g/L glucose. In experiments on this cultivated sludge, Klebsiella (3.7 → 23.0 %) Kosakonia (1.8 → 19.0 %) and Dysgonomonas (0.0 → 15.3 %) sharply increased after lysozyme treatment, due to their fast growth. Treatment with bacteriophage lysozymes can significantly increase sludge reduction.

Performance assessment of a modified trickling filter and conventional activated sludge process along with tertiary treatment in removing emerging pollutants from urban sewage

The absence of effective wastewater treatment technology to eliminate emerging pollutants from municipal sewage has become a pressing issue. In this study, the efficacy of a novel modified trickling filter (MTF), conventional activated sludge process (ASP) and two tertiary systems (UV and ozonation) were compared in eliminating antibiotic-resistant bacteria (ARB), antibiotic resistance genes (ARGs) and pharmaceuticals and personal care products (PPCPs) from urban sewage. MTF and ASP resulted in >1 log unit reduction in the abundance of ARB, while for ARGs, the removal was observed in the range of 0.1 to 1.7 log units. In MTF, ARGs were substantially removed in the aerobic zone compared to the anoxic zone. The relative abundance of most of the ARGs either decreased or remained unchanged during MTF and ASP operations. However, the relative abundance of most of the ARGs increased in the secondary sludge generated from ASP. The concentration of PPCPs such as atenolol, sulfamethazine, triclosan, and ranitidine was reduced by MTF by >80 %. Overall, the results indicated that MTF followed by ozonation is the most effective combination for removing emerging contaminants from municipal sewage.

Insight into the effect of calcium carbonate filler on the dewatering performance of simulated pulp & paper mill sludge

The dewatering of large amount of industry sludge is one of the significant environmental problems in the pulp and paper industry. Calcium carbonate fillers are an important component in primary sludge from paper mills. In this paper, the role of precipitated calcium carbonate (PCC) on the dewaterability of simulated mixed paper mill sludge (the mixture of PCC and secondary sludge) without and with FeCl3 conditioning was studied. The results showed that when the PCC content increased from 0 to 600 mg/g dry sludge (DS) without FeCl3 conditioning, the moisture content (MC) and the specific resistance to filtration (SRF) decreased from 89.4% and 3.1 × 1013 m/kg to 66.7% and 2.4 × 1012 m/kg, respectively. However, when the PCC content was high enough (> 444 mg/g DS), the effective moisture content (MC*) changed little and the net sludge solid yield (YN) decreased, indicating that the non-conditioning effect of PCC dominated at high PCC content. Furthermore, increasing PCC content reduced the compressibility coefficient, although the change was insignificant from 1.06 to 0.91. When FeCl3 was used as the chemical conditioner, FeCl3 and PCC had additive effects on improving the sludge dewaterability (such as MC, SRF and compressibility). The improving mechanism of PCC on the sludge dewaterability can be explained by the release of Ca2+ from PCC caused by lower pH, the bridging effect of multivalent cation and the formation of more rigid sludge flocs.

A performance comparison of cyclopentyl methyl ether (CPME) and hexane solvents in oil extraction from sewage sludge for biodiesel production; RSM optimization

A comparative study of oil extraction from secondary dry sludge was performed via Reflux and Folch methods by cyclopentyl methyl ether (CPME) and hexane as green and organic solvents, respectively. Response surface methodology based on Box-Behnken design (RSM-BBD) was utilized to optimize and predict the impact of influential factors, including sludge amount (4–6 g), the volume of solvent (12–16 mL), and extraction time (2–6 h). The maximum oil extraction efficiency was 8.078 % and 4.076 % under the optimal conditions: 5.15 g, 16 mL, 2 h for CPME, and 4.22 g, 11 mL, 2 h for hexane. The raw sludge was characterized by FESEM and XRF analyses, while the extracted oil characteristics were determined by GC-Mass and FTIR methods. As a result, the Folch method by CPME solvent was a more efficient way to extract oil from sewage sludge. Finally, biodiesel was produced via the transesterification double-step process (TDSP) using NaOH/Clinoptilolite-Fe3O4 magnetic catalyst. The production efficiency of biodiesel from the oil extracted by CPME and hexane is 6.94 % and 4.1 %, respectively. The maximum yield of biodiesel production was6.94 % under the optimum reaction conditions: methanol to oil molar ratio of 10:1, catalyst weight of 0.45 wt%, and reaction time of 2.5 h with the help of the electrolysis method.

Supramolecular bioamphiphile facilitated bioemulsification and concomitant treatment of recalcitrant hydrocarbons in petroleum refining industry oily waste

Bioremediation of real-time petroleum refining industry oily waste (PRIOW) is a major challenge due to the poor emulsification potential and oil sludge disintegration efficiency of conventional bioamphiphile molecules. The present study was focused on the design of a covalently engineered supramolecular bioamphiphile complex (SUBC) rich in hydrophobic amino acids for proficient emulsification of hydrocarbons followed by the concomitant degradation of total petroleum hydrocarbons (TPH) in PRIOW using the hydrocarbonoclastic microbial bio-formulation system. The synthesis of SUBC was carried out by pH regulated microbial biosynthesis process and the yield was obtained to be 450.8 mg/g of petroleum oil sludge. The FT-IR and XPS analyses of SUBC revealed the anchoring of hydrophilic moieties of monomeric bioamphiphilic molecules, resulting in the formation of SUBC via covalent interaction. The SUBC was found to be lipoprotein in nature. The maximum loading capacity of SUBC onto surface modified rice hull (SMRH) was achieved to be 45.25 mg/g SMRH at the optimized conditions using RSM-CCD design. The SUBC anchored SMRH was confirmed using SEM, FT-IR, XRD and TGA analyses. The adsorption isotherm models of SUBC onto SMRH were performed. The integrated approach of SUBC-SMRH and hydrocarbonoclastic microbial bio-formulation system, emulsified oil from PRIOW by 92.86 ± 2.26% within 24 h and degraded TPH by 89.25 ± 1.75% within 4 days at the optimum dosage ratio of SUBC-SMRH (0.25 g): PRIOW (1 g): mass of microbial-assisted biocarrier material (0.05 g). The TPH degradation was confirmed by SARA fractional analysis, FT-IR, 1H NMR and GC-MS analyses. The study suggested that the application of covalently engineered SUBC has resulted in the accelerated degradation of real-time PRIOW in a very short duration without any secondary sludge generation. Thus, the SUBC integrated approach can be considered to effectively manage the hydrocarbon contaminants from petroleum refining industries under optimal conditions.

Enhancing two-phase anaerobic digestion of mixture of primary and secondary sludge by adding granular activated carbon (GAC): Evaluating acidogenic and methanogenic efficiency

Although the granular activated carbon (GAC) has been proved to enhance conventional single-phase anaerobic digestion (AD), how it impacts on acidogenic and methanogenic fermentation is still unknown. In this study, GAC was introduced to elevate the efficiency of two-phase AD, with mixture of primary and secondary sludge as substrate. Five dosages: 0, 0.1, 0.3, 0.5 and 0.7 g GAC/g TSS (Total Suspended Solids) were investigated to determine influences of GAC. The variations of biogas (hydrogen and methane), volatile fatty acids (VFAs), organics degradation and transformation in extracellular polymeric substances (EPS) and dissolved organic matters (DOM) were analyzed. Modified Gompertz model and first-order reaction equation was applied to analyze the kinetics of biogas yield and VFAs utilization, respectively. Sludge reduction, electrical conductance and pH were also quantified to evaluate the system performance. The results showed that GAC could improve two-phase AD performance by enhancing methane production and organics conversion.

Emerging nanotechnology based advanced techniques for wastewater treatment.

The increasing trend of industrialization leads to tremendous release of industrial effluents. Waste water treatment is one of the important sectors to focus in order to overcome the most threatening issue of waste disposal and to ensure sustainability. Sustainable and energy efficient treatment methods are the attractive technologies for their current implementation of waste management. Even though the existing technologies are effective, unsustainability makes them unfit for their extended applications. Conventional and advanced technologies have been extensively implemented for the treatment of wide spectrum of effluents. Hybrid technologies including chemical and biological methods also emerging as promising technologies but secondary sludge generation is still unaddressed. Even though effectiveness of biochar varies over type of contaminants, cost-effectiveness and eco-friendly nature extended their applications in waste management. Nanotechnology and membrane technology are the promising and emerging areas of interest due to their widespread applications in waste water treatment. Carbon nano structures, nano filters, graphene, nano magnets modified with activated carbon are the potential candidates for the treatment. The present review demonstrates the emerging treatment technologies with special focus to nano based waste water treatment methods.

A Laboratory evaluation of Paper mill waste in manufacturing of Clay Bricks

India has one of the world's fastest expanding pulp and paper markets. With a growth rate of more than 10% per year in terms of per-capita growth, The pulp and paper industry requires a lot of capital investment, energy, and water. A paper mill produces 40 to 50 kg of dry sludge (70 percent primary sludge and 30 percent secondary sludge) when 1 tonne of paper is produced. Most of this sludge is now disposed of by land spreading and filling, both of which involve high disposal costs. Pulp and paper industry is one of the most polluted industries in the world in general and India in particular. India produces 10.11 million tons paper per annum which is 2.6% of world’s overall production quantity of paper. This study present using paper mill waste as a component in the manufacture of burnt bricks as a possible route to environmentally friendly construction. This research report details the findings of an experimental investigation into the production of eco-friendly, bricks using soil and paper mills. This research exhibits some increased compressive strength in mix of papermill waste.

Efficient activation of peracetic acid by mixed sludge derived biochar: Critical role of persistent free radicals

Peracetic acid (PAA)-based advanced oxidation processes (AOPs) were increasingly identified as the alternative scheme in wastewater treatment. Cost-effective and easily available catalyst for activation of PAA was in urgent demand for promoting engineering application process. In this study, a new type of biochar catalyst derived from pyrolysis of mixture of primary sludge (PSD) and secondary sludge (SSD) was prepared and showed effective PAA activation ability. The degradation of p-chlorophenol (4-CP) improved with PAA activation by mixed sludge derived biochar (PS-SDBC) than secondary sludge derived biochar (S-SDBC) and primary sludge derived biochar (P-SDBC), and the highest removal efficiency achieved by PS-SDBC with the PSD/SSD ratio of 5/5 (kobs=0.057 1/(M·min), pH 9). Correlation analysis firstly indicated that persistent free radicals (PFRs) rather than chemical composition and material structure dominated PAA activation and organic radicals (RO•) was proved to be the major reactive species through electron paramagnetic resonance (EPR) detection. The mixture of PSD and SSD caused the synergy of inorganic metals and organic matters through pyrolysis processes, resulting in larger specific surface area (SSA) (110.71 m2/g), more abundant electron-donating groups (e.g., C = O, -OH) and massive defects (ID/IG = 1.519) of PS-SDBC than P-SDBC and S-SDBC, which eventually promoted PFRs formation. A fascinating phenomenon was observed in PS-SDBC/PAA system that the active sites of PFRs could be regenerated by RO• attacking onto PS-SDBC, which contributed to the wide pH applicability and continuous stability of PS-SDBC/PAA system in practical wastewater treatment. This study not only significantly deepened the understanding of the reaction mechanism between PAA and biochar, but also provided a potential PAA-based AOPs for micropollutants removal in wastewater.

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.

Energy, exergy, economic and environmental analysis of a running integrated anaerobic digester-combined heat and power system in a municipal wastewater treatment plant

4E (energy, exergy, economic, and environmental) analysis of a municipal wastewater treatment plant (WWTP) including an integrated system with anaerobic digesters and combined heat and power units (AD-CHP) is performed here. This type of analysis was required for AD-CHP system at Tehran WWTP to estimate the plant energy/exergy efficiency, to estimate annual profit and payback period, as well as to study these parameters with change in plant operating conditions. This necessary type of analysis was not found in literature and in previous studies. The results showed that the integrated system generated 4.6 MWel,net and 2.1 MWth,net with 3100 m3/day of total mixture of primary and secondary sludge (4.5% total solids and 81.2% volatile solids fraction). In this regard, the system energy efficiency was 24.3%. The flow exergy values throughout the plant were also estimated and demonstrated in two Sankey and Grassmann diagrams which showed the plant overall exergy efficiency of 17.3%. The integrated system also reduced CO2 emissions by 64% compared to that for a traditional energy supply system (buying electricity from the grid and producing hot water by burning biogas produced from anaerobic digesters). In economic analysis, a selling incentive scenario employed in which generated electricity was sold to the grid at price of 0.125 $/kWh while the price of electricity purchased was 0.081 $/kWh. The payback period was 2.4 years in this case which was significantly lower (57%) than that for the case with no-selling electricity to the grid (5.6 years). Moreover, the levelized cost of electricity (LCOE, which is the minimum selling price of electricity required by the integrated system or the point at which cost and revenue are equal) was 127 $/MWh, which was within the acceptable range reported by the International Renewable Energy Agency for the biomass-power generation technologies.

Feasibility of Thermal Utilization of Primary and Secondary Sludge from a Biological Wastewater Treatment Plant in Kaliningrad City

Hydrothermal liquefaction (HTL) of sewage sludge is considered in the article as an analogue of the natural processes of oil formation (catagenesis). A comparison of the physicochemical composition of primary and secondary sludge with type II kerogen (natural precursor of oil) showed their similarity. Both types of sludge have a slightly higher level of oxygen and nitrogen. The study tested the hypothesis that the elements included in the inorganic part of the oil source rocks can have a catalytic effect on the oil formation processes. For the conducted studies of sludge HTL, the catalysts containing cations and substances found in oil source rocks were chosen: as homogeneous catalysts (KOH, NaOH, NH4Fe(SO4)2, CoCl6, NiSO4, CuSO4, ZnSO4, MoO3) and as heterogeneous catalysts (MgO, Zeolite, Al2O3). The effectiveness of catalysts containing metal ions, zeolite and aluminum oxide has been proven. The highest biocrude yield was achieved in a process with NiSO4 as the catalyst in a dose 2 g per 10 g of sludge: oil yield increased by 34.9% and 63.4% in the processing of primary and secondary sludge, respectively. The use of catalysts provided an increase in fuel HHV by 10.8–12.5%, which is associated with a decrease in oxygen content (by 10.8–43.2%) with a simultaneous increase in carbon (by 7.9–10.9%) and hydrogen (by 6.5–18.7%) content.

Valorization of wastewater through microalgae as a prospect for generation of biofuel and high-value products

Environmental pollution and freshwater crisis are the main pushing forces for upgrading new insights in wastewater treatment . Wastewater generated from different sources is an abundant source of various nutrients; their recovery could be a better perspective to compensate for the demands of the increasing population. Physicochemical or bacterial wastewater treatment processes are inefficient in removing nitrogen (N) and phosphorous (P), are highly energy demanding, and generate secondary sludge . Microalgae offer a better alternative in wastewater treatment because they can consume different inorganic components in wastewater, including nitrogen, phosphorus, and some metals for growth and biomass production . They accomplish pollutants removal coupled with the production of valuable biomass for the generation of bioenergy and commercial products in an eco-friendly manner. The present review focuses on the details of different types of wastewater along with various challenges of microalgal cultivation systems applied in wastewater treatment. The improvements and recent advances in the remediation of wastewater involving microalgae are discussed. Different microalgal growth models and photorespirometry to assess the overall functioning of microalgae in wastewater are also summarized. Furthermore, various industrially important products that can be sustainably obtained from microalgal biomass through wastewater bioremediation are also highlighted. Finally, future perspectives in this field are presented. • In-depth insights into wastewaters bioremediation through microalgae are reviewed. • Mechanisms involved in the microalgal bioremediation of wastewater are discussed. • Challenges and prospects of microalgae-based treatment processes are presented. • Such dual benefit techniques serve towards bioremediation and value added products.

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.

Effects of hydrothermal treatment on organic compositions, structural properties, dewatering and biogas production of raw and digested sludge

The effects of hydrothermal treatment (HTT) under different temperatures and time (120 °C to 250 °C, 10 min to 60 min) on organic matter solubilization and structure changes of secondary sludge (SS) and digested sludge (DS), as well as downstream dewatering and anaerobic digestion were investigated. The results showed that organic matter solubilization increased significantly at 120 °C to 170 °C, then decreased at 200 °C to 250 °C. The organic matter solubilization during HTT showed no obvious difference for two sludge, but for the different organic components. The polysaccharides are easier to be dissolved than protein, which was manifested by the higher dissolution rate at low temperature. The protein was the main soluble component for both of hydrothermal SS and DS, which accounted for 44 % to 64 % of soluble chemical oxygen demand (SCOD). The decrease of residual extracellular polymeric substances (EPS) content and increase of N-acetylglucosamine and DNA concentrations indicated that sludge EPS and cell wall structure were damaged at 170 °C, which contributed to the high organic matter solubilization. Nitrogen balance and molecular weight distribution indicated the concentrations of soluble organic components were the combined result of dissolution and hydrolysis reaction. The hydrolysis and polymerization reaction were intensified at 170 °C to 250 °C, which was verified by the COD balance and molecular weight transformation. The hydrothermal time could further facilitate the organics dissolution and hydrolysis based on the effect of hydrothermal temperature. The EPS structure damage also contributed to the high percentage of free moisture, resulting in enhanced dewaterability. The highest methane production was 298.1 mL CH4/g VSadd for DS hydrothermally treated at 170 °C, which were 125 % and 9.8 % higher than SS and SS-HTT, respectively. This study provided an insight into the general mechanism of HTT and the application of different HTT and AD configurations.

Bio-gas augmentation and waste minimization by co-digestion process in anaerobic digestion system of a municipal waste water treatment plant

Anaerobic digesters of Tehran waste water treatment plant (WWTP) working in mono-digestion operation are investigated for not only methane augmentation but also for waste minimization by the use of co-digestion process. Three types of co-substrate wastes including slaughterhouse flotation greases, biowaste, and cattle manure were selected because they were available in large amounts. A model was required to predict the volume flow rates of feeding co-substrate wastes into digesters during operation time for providing sufficient biogas in order to generate maximum gas engine power output. Thus, Anaerobic Digestion Model Number 1 (ADM1) for mono-substrate is modified so that it could be used with characterization of primary sludge, secondary sludge, and several co-substrate waste inputs into digesters. Then, appropriate coefficients (hydrolysis coefficient and bio-degradability factor) in this model were determined by the use of sludge biochemical methane potential (BMP) tests. After complete modification, calibration and validation of ADM1 method with experimental values of primary and secondary sludge for anaerobic digestion (which is the current reality in Tehran WWTP), the developed model is integrated with a co-substrate feeding strategy for estimation of required feed flow rates of co-substrate wastes. Results indicated that feeding grease, biowaste and manure increased the average generated biogas from 10524 m3/day (62.1% CH4) to 29161 m3/day (61.5% CH4), 30183 m3/day (59.5% CH4), and 32531 m3/day (53.8% CH4), respectively. Also, the gas engine power output increased from an average value of 906 kW to about 2.5 MW (during the studied operation time period). Low hydrolysis coefficient and low degradability of manure caused high bio-degradable feed flow rate of this co-substrate waste to about 58,200 kgCODb/day in average. This value for biowaste and grease was 38,800 and 36,100 kgCODb/day, respectively. Feeding grease and biowaste into digesters increased COD removal efficiency from 37% to 58% and 54%, respectively, while feeding manure dropped it to 31%. Furthermore, values of pH and VFA/Alk indicators showed stable conditions in digesters. Moreover, it was found that all three co-digestion processes did not impose a new load on other WWTP equipment for reducing nitrogen in the effluent stream of WWTP. Finally, it was concluded that biowaste is an appropriate co-substrate for our case study.