Municipal Wastewater Sludge Research Articles

Influence of wastewater sludge properties on the performance of electro-osmosis dewatering

ABSTRACTAlthough the properties of municipal wastewater sludge play key roles in the electro-osmosis dewatering process, it is still controversial which properties have the greatest effect on the dewatering performance. In this study, multiple regression models with the Group Lasso method were used to investigate the relationship between the final moisture content and the sludge properties, including pH, electrical conductivity (EC), volatile solids content, zeta potential (ζ), initial moisture content, extracellular polymeric substances (EPS), proteins of EPS (EPSPr), polysaccharides of EPS (EPSPo) and the ratio of EPSPr and EPSPo (EPSR). Under the optimal conditions (pressure = 100 kPa, voltage = 50 V and cake thickness = 15 mm), EPS, EC and ζ were significantly related to sludge dewaterability and EPS was the most important factor. Furthermore, the coefficient estimate of EPSPo was greater than that of EPSPr and the coefficient of EPSR was negative, indicating that EPSPo plays more important roles in electro-osmosis dewatering than EPSPr. Thus, reducing the EPS content of sludge, especially the EPSPo content, is necessary to improve the performance of electro-osmosis dewatering.

Bench-Scale Evaluation of Hydrothermal Processing Technology for Conversion of Wastewater Solids to Fuels.

Hydrothermal Liquefaction (HTL) and Catalytic Hydrothermal Gasification (CHG) proof-of-concept bench-scale tests were performed to assess the potential of hydrothermal treatment for handling municipal wastewater sludge. HTL tests were conducted at 300 to 350 °C and 20 MPa on three different feeds: primary sludge, secondary sludge, and digested solids. Corresponding CHG tests were conducted at 350 °C and 20 MPa on the HTL aqueous phase output using a ruthenium-based catalyst. Biocrude yields ranged from 25 to 37%. Biocrude composition and quality were comparable to biocrudes generated from algae feeds. Subsequent hydrotreating of biocrude resulted in a product with comparable physical and chemical properties to crude oil. CHG product gas methane yields on a carbon basis ranged from 47 to 64%. Siloxane concentrations in the CHG product gas were below engine limits. The HTL-CHG process resulted in a chemical oxygen demand (COD) reduction of > 99.9% and a reduction in residual solids for disposal of 94 to 99%.

Application of dielectric constant measurement in microwave sludge disintegration and wastewater purification processes.

It has been numerously verified that microwave radiation could be advantageous as a pre-treatment for enhanced disintegration of sludge. Very few data related to the dielectric parameters of wastewater of different origins are available; therefore, the objective of our work was to measure the dielectric constant of municipal and meat industrial wastewater during a continuous flow operating microwave process. Determination of the dielectric constant and its change during wastewater and sludge processing make it possible to decide on the applicability of dielectric measurements for detecting the organic matter removal efficiency of wastewater purification process or disintegration degree of sludge. With the measurement of dielectric constant as a function of temperature, total solids (TS) content and microwave specific process parameters regression models were developed. Our results verified that in the case of municipal wastewater sludge, the TS content has a significant effect on the dielectric constant and disintegration degree (DD), as does the temperature. The dielectric constant has a decreasing tendency with increasing temperature for wastewater sludge of low TS content, but an adverse effect was found for samples with high TS and organic matter contents. DD of meat processing wastewater sludge was influenced significantly by the volumetric flow rate and power level, as process parameters of continuously flow microwave pre-treatments. It can be concluded that the disintegration process of food industry sludge can be detected by dielectric constant measurements. From technical purposes the applicability of dielectric measurements was tested in the purification process of municipal wastewater, as well. Determination of dielectric behaviour was a sensitive method to detect the purification degree of municipal wastewater.

Phenol-degrading sludge as a promising precursor for a capacitive carbon material: Disclosing key factors for the nanostructure and high capacitance

The use of sludge, a side product of wastewater treatment, as a precursor for the synthesis of high-capacitance carbon is environmentally benign; however, the key factors influencing the structure of the carbon, and accordingly, its capacitive performance, remain unknown. We demonstrate that the physicochemical and electrochemical properties are remarkably different among four different sludge-derived carbons: those obtained from municipal wastewater sludge, coking wastewater sludge, phenol-acclimated municipal wastewater sludge, and phenol-acclimated coking wastewater sludge (denoted as MSC, CSC, P-MSC, and P-CSC, respectively). Hierarchical mesoporous graphene-like carbon was obtained from the acclimated sludge (particularly for P-CSC) with a high surface area, sufficient degree of graphitization, good electrical conductivity, and rich N and P doping. Accordingly, the P-CSC electrode delivered a large specific capacitance (416.1 F g−1 at 1 A g−1), good rate capability (retention ratio of 80.7% from 1 to 15 A g−1), and long-term cycling stability (retention ratio of 96.9% after 10,000 cycles), outperforming the MSC, CSC, and P-MSC electrodes. After conducting a series of control experiments, we determined that the contents of Fe, N, and P in the sludge, and the affinity of the microorganisms and particles/flocs in the sludge are crucial to the final characteristics of the sludge-derived carbon.

Development of bioelectrochemical systems using various biogas fermenter effluents as inocula and municipal waste liquor as adapting substrate

The purpose of this research was to improve microbial fuel cell (MFC) performance – treating landfill-derived waste liquor – by applying effluents of various biogas fermenters as inocula. It turned out that the differences of initial microbial community profiles notably influenced the efficiency of MFCs. In fact, the adaptation time (during 3 weeks of operation) has varied significantly, depending on the source of inoculum and accordingly, the obtainable cumulative energy yields were also greatly affected (65% enhancement in case of municipal wastewater sludge inoculum compared to sugar factory waste sludge inoculum). Hence, it could be concluded that the capacity of MFCs to utilize the complex feedstock was heavily dependent on biological factors such as the origin/history of inoculum, the microbial composition as well as proper acclimation period. Therefore, these parameters should be of primary concerns for adequate process design to efficiently generate electricity with microbial fuel cells.

Anaerobic Digestion Effluents (ADEs) Treatment Coupling with Chlorella sp. Microalgae Production.

Nutrient removal effectiveness from anaerobic digestion effluents (ADEs) by Chlorella sp. cultivation and microalgae biomass productivity were evaluated in this study. The results showed that the highest Chlorella sp. biomass productivities of 386.5 ± 24.1 mg dry weight/L•d and 338.3 ± 11.0 mg dry weight/L•d were respectively obtained with the anaerobically digested effluent of municipal wastewater sludge and effluent from a fermentation tank treating dairy wastewater. Lower (p < 0.05) microalgal growth was achieved with anaerobically digested effluents of maize silage and swine slurry and cattle manure. The increase of the initial ammonia nitrogen concentration in ADEs to the level of 160 mg/L did not encourage Chlorella sp. productivity because of phosphorus limitation. The removal efficiencies of ammonia nitrogen, total nitrogen, total phosphorus, and chemical oxygen demand (COD) reached 99.7%, 98.6%, 88.2%, and 58.7%, respectively, depending on the source of ADE, but not on the initial ammonia nitrogen concentrations.

Recovery of polyhydroxyalkanoates from municipal secondary wastewater sludge

In the current study, the feasibility of utilizing municipal secondary wastewater sludge for Polyhydroxyalkanoate (PHA) extraction was improved by optimization of various parameters (temperature, duration and concentration of sludge solids). Optimized process parameters resulted in PHA recovery of 0.605 g, significantly higher than un-optimized conditions. The characterization of PHA was carried out by GC–MS, FT-IR and NMR (1H and 13C) spectroscopy. The PHA profile was found to be dominated by mcl PHA (58%) along with other diverse PHA. The results of the present study show rich diversity of PHA extracted from a raw material which is readily available at minimal cost. In conclusion, exploring the potential of wastes for production of bioplastics not only reduces the cost of bioplastic production, but also provides a sustainable means for waste management.

Seasonal variation of elements composition and biomethane in brown macroalgae

To investigate the effect of seasonal variation on macroalgae biomass characteristics and its related energy content, four different algae species, two from the Fucales order (Fucus vesiculosus and Ascophyllum nodosum), and two from the Laminariales order (Saccharina latissima and Alaria esculenta), were harvested during spring, summer, autumn and winter over one year. The biomethane potential and the elemental composition were determined for all samples. Both Fucales species showed low biodegradability (<30%) with methane yields consistently below 20 Nm3 per wet tonne. Laminariales species, however, allowed up to 80% biodegradation efficiency, with methane yields varying between 20 and 70 Nm3 per wet tonne depending on the harvesting season. For each of the four algae the biomass concentrations of potassium, phosphorus, calcium, magnesium, sodium, sulphur, aluminium and iron was higher during spring/summer compared to autumn/winter. For heavy metals, variation was seen mainly for arsenic with higher values during autumn/winter. The highest values were observed for Laminariales (30–120 mg/kg total solids). Modelling the co-digestion of each algae with municipal wastewater sludge showed Laminariales species having the greatest potential as a profitable substrate for additional biomethane production generating up to 0.38 million Euro. However, seasonal variation could affect this expected income over 60% due to fluctuation in biomethane yields and biodegradability efficiency. Furthermore, the additional loading of cadmium and arsenic into the AD process suggested possible limitation for digestate utilisation in arable lands.

Release characteristics and control of hydrogen sulfide during thermal drying of municipal wastewater sludge

Hydrogen sulfide (H2S) is the most predominant malodorous gas released during the thermal drying of municipal wastewater sludge. Experiment using a laboratory-scale tube furnace was conducted to determine the characteristics of H2S release and the changes of sludge moisture content. The addition of calcium oxide (CaO) in the original sludge was also tested and evaluated for its inhibitory effect on the H2S release. The results showed that the amount of H2S released increases with the increase of temperature in the tested range of temperature from 80 to 280 °C. The critical temperature range defined as the turn point of temperature at which the H2S or other odorous gases release starts to drastically increase was determined to be 200–240 °C. It was found that the drying process could be divided into two stages according to the changes of H2S release speed and the corresponding moisture content, and the majority of H2S release was detected in the second stage. The reduction of element sulfur in sludge was approximately proportional to the H2S released in the temperature range from 80 to 280 °C. The addition of CaO in the sludge could significantly repress the release of H2S during thermal drying. This inhibitory effect was observed to last for the entire drying process and not be changed with the change of temperature.

Biogas production from anaerobic digestion of food waste and relevant air quality implications

ABSTRACTBiopower can diversify energy supply and improve energy resiliency. Increases in biopower production from sustainable biomass can provide many economic and environmental benefits. For example, increasing biogas production through anaerobic digestion of food waste would increase the use of renewable fuels throughout California and add to its renewables portfolio. Although a biopower project will produce renewable energy, the process of producing bioenergy should harmonize with the goal of protecting public health. Meeting air emission requirements is paramount to the successful implementation of any biopower project. A case study was conducted by collecting field data from a wastewater treatment plant that employs anaerobic codigestion of fats, oils, and grease (FOG), food waste, and wastewater sludge, and also uses an internal combustion (IC) engine to generate biopower using the biogas. This research project generated scientific information on (a) quality and quantity of biogas from anaerobic codigestion of food waste and municipal wastewater sludge, (b) levels of contaminants in raw biogas that may affect beneficial uses of the biogas, (c) removal of the contaminants by the biogas conditioning systems, (d) emissions of NOx, SO2, CO, CO2, and methane, and (e) types and levels of air toxics present in the exhausts of the IC engine fueled by the biogas. The information is valuable to those who consider similar operations (i.e., co-digestion of food waste with municipal wastewater sludge and power generation using the produced biogas) and to support rulemaking decisions with regards to air quality issues for such applications.Implications: Full-scale operation of anaerobic codigestion of food waste with municipal sludge is viable, but it is still new. There is a lack of readily available scientific information on the quality of raw biogas, as well as on potential emissions from power generation using this biogas. This research developed scientific information with regard to quality and quantity of biogas from anaerobic co-digestion of food waste and municipal wastewater sludge, as well as impacts on air quality from biopower generation using this biogas. The need and performance of conditioning/pretreatment systems for biopower generation were also assessed.

Iron removal in highly contaminated acid mine drainage using passive biochemical reactors.

Passive biochemical reactors (PBRs) are a viable alternative to neutralization plants for the treatment of acid mine drainage (AMD) because they require lower investment costs and use residual materials. However, high iron (Fe) concentrations (≥0.5 g/L) in AMD are challenging for their long-term efficiency. Sorption and precipitation are the main Fe removal mechanisms, but the relative importance of each is mostly unknown. In this study, locally available natural materials (organic and inorganic) were characterized and tested for their performance in Fe removal from highly contaminated AMD (pH 3.5, 4 g/L of Fe, and 9 g/L of sulfate). Iron retention capacity of the materials was then evaluated and the efficiency of eight mixtures of materials was compared through 40-day laboratory batch tests. All batch-type PBRs increased the pH up to 6.5 and decreased dissolved metals concentrations, including Fe, up to 99%. Results showed that organic residual materials (manures, municipal wastewater sludge, and compost) were the best substrates for Fe removal.These findings allowed for the selection of three reactive mixtures with distinct characteristics (mixture #1 - 30% organic wastes; mixture #4 - 50% calcite; and mixture #7 - 50% sand) to be further evaluated in column type PBRs.

Archaeal Diversity in a Municipal Wastewater Sludge

The diversity of Archaea in a municipal wastewater sludge sample was investigated by amplification of ribosomal RNA genes from sludge DNA using archaeal-specific primers. Surprisingly, a large fraction (32%) of these sequences were from Halobacteriales, not previously seen in surveys of wastewater sludge. Other abundant sequences were from members of uncultivated ‘environmental’ archaeal groups that are commonly detected in sludge and sediment environments. Only a few distant relatives of Methanosarcina (which are commonly thought of as the predominant methanogenic species in sludge environments) were detected.

Heavy metal content of Swedish municipal wastewater sludge - status and goals.

In 2014, 25% of the sludge produced at Swedish municipal wastewater treatment plants was applied to agricultural land. Even though the Swedish heavy metal limits for sludge to be used in agriculture are amongst the most stringent in the EU, more stringent heavy metal limits are proposed. Most sludge applied to agricultural land is recycled within a certification system, Revaq. Revaq has targets for control at source management and improvement of sludge quality. Statistics based on data collected within the Revaq system was used to differentiate between local and general sources of heavy metals and assess the need to improve sludge quality. The analysis indicates that proposed future national limits on the quality of the sludge can be met by most of the sludge. The improvement needed for about 20% of the sludge is feasible through local control at source management. The levels of cadmium, copper and mercury need to be reduced if these metals are not to limit the amount of sludge that may be applied per unit area of arable land. Finally, the long term Revaq targets for cadmium and silver will be difficult to meet.

Municipal wastewater sludge as a sustainable bioresource in the United States

Within the United States and Puerto Rico, publicly owned treatment works (POTWs) process 130.5 Gl/d (34.5 Bgal/d) of wastewater, producing sludge as a waste product. Emerging technologies offer novel waste-to-energy pathways through whole sludge conversion into biofuels. Assessing the feasibility, scalability and tradeoffs of various energy conversion pathways is difficult in the absence of highly spatially resolved estimates of sludge production. In this study, average wastewater solids concentrations and removal rates, and site specific daily average influent flow are used to estimate site specific annual sludge production on a dry weight basis for >15,000 POTWs. Current beneficial uses, regional production hotspots and feedstock aggregation potential are also assessed. Analyses indicate 1) POTWs capture 12.56 Tg/y (13.84 MT/y) of dry solids; 2) 50% are not beneficially utilized, and 3) POTWs can support seven regions that aggregate >910 Mg/d (1000 T/d) of sludge within a travel distance of 100 km.

Evaluation of Autothermal Thermophilic Aerobic Digester Performance for the Stabilization of Municipal Wastewater Sludge.

Sludge stabilization process in terms of operational, environmental and economic indexes is the most important stage of treatment and its disposal. This study was aimed to determine the performance of Autothermal Thermophilic Aerobic Digestion (ATAD) system as one of the low-cost and biocompatible methods of sludge treatment. This study has been done using a laboratory scale Autothermal Thermophilic Aerobic Digestion (ATAD). The reactor was consisted of two polyethylene tanks with a final capacity of 100 L for each tank. Both tanks with all fittings were installed on a metal frame. The variables of study were temperature, dissolved oxygen, pH, volatile organic compounds, total solids, COD and the number of Ascaris eggs and fecal coliforms per gram of dry matter of the sludge. The temperature was measured hourly and the pH and dissolved oxygen were measured and controlled twice per day. One-way ANNOVA was applied to analyze reasults. According to the results, the temperature of sludge increased from 11.7-61.2°C by biological reactions. Pathogen organisms were reduced from 80×106 to 503 in number during 72 h. After 6 days pathogen organisms and Ascaris eggs were removed completely. Volatile organic compounds and COD were reduced 42 and 38.3% respectively during the 6 days. It is concluded that the performance of ATAD in removing organic compounds from wastewater sludge were desirable. Resulted sludge from stabilization process were appropriate for use in agriculture as a soil supplement and met the indexes of class A sludge according to EPA's standards (CFR 40 Part 503).

Inclusion of Saccharina latissima in conventional anaerobic digestion systems

ABSTRACTLoading macroalgae into existing anaerobic digestion (AD) plants allows us to overcome challenges such as low digestion efficiencies, trace elements limitation, excessive salinity levels and accumulation of volatile fatty acids (VFAs), observed while digesting algae as a single substrate. In this work, the co-digestion of the brown macroalgae Saccharina latissima with mixed municipal wastewater sludge (WWS) was investigated in mesophilic and thermophilic conditions. The hydraulic retention time (HRT) and the organic loading rate (OLR) were fixed at 19 days and 2.1 g l−1 d−1 of volatile solids (VS), respectively. Initially, WWS was digested alone. Subsequently, a percentage of the total OLR (20%, 50% and finally 80%) was replaced by S. latissima biomass. Optimal digestion conditions were observed at medium-low algae loading (≤50% of total OLR) with an average methane yield close to and in mesophilic and thermophilic conditions, respectively. The conductivity values increased with the algae loading without inhibiting the digestion process. The viscosities of the reactor sludges revealed decreasing values with reduced WWS loading at both temperatures, enhancing mixing properties.

Forecasting and analysis of biogas-based power production using extremal neural network

ABSTRACTThe primary objective of this study is to design an efficient and robust method to predict the power production from the anaerobic digester by taking into consideration various factors and feedstocks (organic fraction municipal solid waste, wastewater sludge, and co-digestion). This study focuses on the influence of primary factors such as temperature, pH, and nitrogen concentration on power production. Extremal neural network tool was developed with the assistance of MATLAB programming to predict the power production. The results showed that this approach is trustworthy enough to predict the energy output with respect to the primary variables mentioned above. It is expected that this new prediction model will improve the biogas productivity and contribute significantly to energy production. Moreover, this design has an economic advantage in processing.

Optimization of ultrasonic waves application in municipal wastewater sludge treatment using response surface method

Today, many limitations are faced in sludge treatment and disposal. Therefore evaluation of different approaches to reduce sludge production in the activated sludge process has attracted great attention. Application of ultrasonic waves in sludge treatment caused to reduce sludge volume and accelerate sludge digestion. This research intended to study the efficiency of ultrasound in dewatering biological sludge in wastewater treatment plants under different conditions. In this study, response surface method was used to investigate results and optimum conditions were determined. Sludge was treated in different conditions as follows: 330–920 W ultrasound power, 1.5–3.9 L sample volume and 6–20 min ultrasonic exposure duration. Then, the effect of waves was studied in terms of SRF (specific resistance to filtration). Results of the experiments showed that, the ultrasonic method significantly increases the SRF. Also based on response surface method, the best performance of ultrasonic application in sludge treatment is achievable at the following conditions: 625 W ultrasound power, 2.7 L sample volume and 13 min ultrasonic exposure duration. A mathematical model for accurate prediction of SRF changes of the sludge was derived using statistical data.

Influences of low-energy input microwave and ultrasonic pretreatments on single-stage and temperature-phased anaerobic digestion (TPAD) of municipal wastewater sludge

The effect of low energy-input (2.5 kJ/g total solids) microwave (MW) irradiation and ultrasonication (US) on single-stage mesophilic anaerobic digestion (AD) and temperature-phased AD (TPAD) treating wastewater sludge was assessed. The goal was to achieve a higher net energy along with improved digestate for agricultural applications. At sludge retention times (SRT) of 20 and 14 days, TPAD achieved significantly (39–45%) higher methane production, pathogen destruction (4-log) and digester volume reductions compared to single-stage AD (controls). The higher methane yields resulted in higher net energies for TPAD (4.7–5.1 kJ/g volatile solids or VS) compared to controls (3.3–3.7 kJ/g VS). Although improvements in organic removals and methane yields were observed after MW and US, the energy input for pretreatments were not compensated by the increased methane production, and therefore were concluded to be infeasible for industrial applications for single-stage and TPAD. All TPADs provided the highest pathogenic removal, and met the Class A biosolids fecal coliform requirements, while MW pretreatment achieved the highest fecal coliform removal for single-stage AD with digestates classified as Class B. Digesters incorporating thermal pretreatment (MW and MW + TPAD) yielded the highest amount of extra heat (4.2–5.8 kJ/g VS) that could be utilized within/outside of the facility.

Application of dissolved air flotation process for industrial sludge thickening: A laboratory-scale study

Aims: Increasing population density in cities has led to an increase in industrial and municipal wastewater sludge generation volume. Dissolved air flotation (DAF) can widely be used for water and wastewater treatment. The aim of our study is to evaluate some DAF design parameters on biological sludge thickening which generated from an industrial wastewater treatment plant. Materials and Methods: This experimental research had been carried out as a laboratory-scale study on DAF process to thickening of the biological sludge. The required amount of sludge was taken once from the Amikabir industrial town wastewater treatment plant in Kashan, Iran. After determination of optimal recycling rate, pH, and coagulant dose, effects of pressure (3, 5, and 7 atm), flotation time (5 and 10 min), and coagulant addition were evaluated on DAF process efficiency. Results: According to this investigation results, the optimal pH and dose of coagulant were obtained 7.5 and 200 mg/L, respectively. Maximum process efficiency for the reduction of total dissolved solids (TDSs), total solids (TSs), and turbidity parameters was 61.01%, 84.02%, and 97%, respectively. Furthermore, the results showed that in contrast to time, coagulant addition and pressure have significant effect on DAF process. Conclusion: DAF process had suitable thickening efficiency to removal of TDS, TS, and turbidity on biological activated sludge.