Grease Trap Sludge Research Articles

The economic efficiency of the co-digestion at WWTPs: A full-scale study.

The methane and digestate production from biowaste (BW, 95% food waste and 5% garden waste based on fresh mass) and grease trap sludge (GTS) co-digestion at the Grossache-Nord WWTP (Austria) as a basis for a cost-benefit analysis was determined using two approaches: The first one was to determine the specific methane yields (SMY) and total solids (TS) removals (%) of the used substrates in biomethane potential (BMP) tests. In the second, the full-scale process data from a supervisory control and data acquisition (SCADA) system were analyzed. From these data, the SMY of the sewage sludge (SS) was calculated for a period without co-digestion and applied to the study period. Thus, it was possible to calculate the methane and digestate production from the co-substrates. Both approaches produced different co-substrate SMYs and TS degradation results. In the approach using the BMP, the SMY was 518m3/t TSadded and the TS degradation was 77%. For the full-scale method, these values were found to be 620m3/t TSadded and 66%, respectively. However, the cost-benefit analysis of both approaches indicated that electricity generation from co-digestion can cover the associated costs. The benefit to cost ratio was 1.14 and 1.08 for the BMP and full-scale approach, respectively. The application of the respective approach depends on the availability and quality of full-scale process SCADA data.

Biomethane Potential of Selected Organic Waste and Sewage Sludge at Different Temperature Regimes

Sewage sludge (SS) generation and its management still pose a problem in many countries. Anaerobic co-digestion (AcD) of SS with grease trap sludge (GTS) and organic fraction of municipal sewage waste (OFMSW), which are two easily biodegradable substrates, may improve biogas production and AcD kinetics. Algae biomass (AB) of the species Undaria pinnatifida can be the third co-digestion component that may also affect the AcD performance. The aim of the study was therefore to evaluate the performance of mesophilic and thermophilic SS batch AcD with OFMSW, GTS as well as AB through biochemical methane potential (BMP) assay in relation to cumulative specific biogas (YB) and methane yields (Ym). Three kinetic models were applied within the scope of the kinetic study. Results of the study showed that the mixture containing SS, GTS and AB brought the most noticeable improvements in Ym compared to other studied mixtures and in respect to standalone SS digestion, the improvement amounted to 88.37% at mesophilic temperature (260.83 ± 15.02 N mL CH4/g-VSadd and for standalone SS 138.47 ± 4.70 N mL CH4/g-VSadd) and 71.09%, respectively, at the thermophilic one (275.66 ± 4.11 N mL-CH4/g-VSadd and for SS standalone 161.13 ± 13.11 N mL-CH4/g-VSadd).

The influence of grease trap sludge sterilization on the performance of anaerobic co-digestion of sewage sludge

Abstract The effect of sterilization of grease trap sludge on the performance of their joint anaerobic digestion with sewage sludge was investigated. The process was carried out in two continuously stirred-tank glass reactors at mesophilic conditions. The hydraulic retention time was 10 days, and the experiment was divided into three stages. In two stages, anaerobic digestion of sewage sludge was carried out. In the first stage, the batch assay was applied to promote the development of an anaerobic community. In the second stage, the reactors were fed with sewage sludge in a semi-continuous regime. Meanwhile, in the next step, grease trap sludge was co-digested with sewage sludge. For the first period of the 3rd stage, grease trap sludge without sterilization was dosed into feedstock. In the last period, waste prior to introduction to the feedstock, was sterilized at 133 °C for 20 min at a steam pressure equal to 3 bars. The results of the statistical analyses revealed that the addition of co-substrates to the feedstock had a significant influence on biogas yield, as well as volatile solids removal. However, the conditioning of the grease trap sludge did not have a significant statistical effect on the production of biogas (enhanced by 3%) and volatile solids removal. Moreover, there were no statistically significant changes in the composition of biogas.

Benefits and drawbacks of food and dairy waste co-digestion at a high organic loading rate: A Moosburg WWTP case study

Anaerobic co-digestion (AcoD) is a key technology in reframing organic waste as a viable energy source. A lack of documented experience on full-scale AcoD at wastewater treatment plants (WWTPs) has created a bottleneck in AcoD implementation, which is further tightened by the focus of existing AcoD studies being on low co-substrate loading (<50%) and the obtainable benefits. This study aims to fill this gap by investigating the drawbacks and benefits of high-ratio co-substrate dosing of food and dairy wastes at the Moosburg WWTP (Germany) from 2014 to 2017. The Moosburg WWTP co-digests sewage sludge, food waste, and dairy wastes at a 35:47:18 ratio by volatile solids (organic loading rate (OLR) of 3.0 kgVS/(m3·day)). During the study period, this high co-substrate dosing increased the methane potential by 300 ± 50%. The corresponding high methane yield significantly increased the on-site electricity production, resulting in energy neutrality in 2014–2015. The corresponding economic gain from gate fees was 48,000 ± 5,000 € per year. The observed drawbacks included solids accumulation inside the digester (5 m3/month), high nitrogen backload (65% increase from co-substrate addition), reduced retention time (loss of 1.18 days/year from solids accumulation), and reduced dewaterability. The high nitrogen content in the centrate is treated by sequential batch reactors (SBRs), using lactose as the carbon source for denitrification. This study presents an alternative approach for determining gate fees based on the economic gains from inherent methane content, which identified waste milk, lactose and grease trap sludge as the most profitable co-substrates.

Determination of methane potential of mixtures composed of sewage sludge, organic fraction of municipal waste and grease trap sludge using biochemical methane potential assays. A comparison of BMP tests and semi-continuous trial results

The main objective of this study was to evaluate the effect of simultaneous addition of grease trap sludge and organic fraction of municipal waste on anaerobic digestion of sewage sludge. Batch experiments were performed under mesophilic conditions to determine the biochemical methane potential and methane production rate of raw material as well as their mixtures. The optimum mixing ratio of sewage sludge with grease trap sludge and organic fraction of municipal waste was 4:3:3. For this mixture methane yield was 327 LCH4/kg VS, which is higher by approximately 130% than that of sewage sludge alone. Additionally, selected models were used to analyze the kinetics of anaerobic co-digestion.Despite the significant differences in the methane yields reported for the BMP assays and the semi-continuous trial, for the estimated profit of thermal energy and the theoretical thermal/electric power production for both experimental procedures similar trends were observed (values increasing with the gradual increase in the share of the co-substrate in the mixtures). Moreover the estimated degree in which electric energy obtained from the combustion of biogas covers the energy demand of the wastewater treatment plant showed that the addition of both co-substrates may lead to a value of 125% for this parameter.

The influence of decreased hydraulic retention time on the performance and stability of co-digestion of sewage sludge with grease trap sludge and organic fraction of municipal waste

The effect of hydraulic retention time ranging from 12 to 20 d on process performance and stability was investigated in two anaerobic completely stirred tank reactors with a working liquid volume equal to 6 litres. The reactors were fed with mixtures containing (on volatile solids basis): 40% of sewage sludge, 30% of organic fraction of municipal waste and 30% of grease trap sludge. The change of hydraulic retention time did not significantly affect process stability. However, methane yields as well as volatile solids removal decreased from 0.54 to 0.47 l per kg of added volatile solids and 65% to 60% respectively, with the decrease of hydraulic retention time. Despite the fact that the best process performance was achieved for hydraulic retention time of 20 days, the obtained results showed that it is also possible to carry out the co-digestion process at shorter hydraulic retention times with good results. Furthermore, gas production rate as well as biogas production at the shortest hydraulic retention time were approximately 46% higher in comparison to results obtained at the longest hydraulic retention time. In this context, the proposed solution seems to be an interesting option, because it provides an unique opportunity for wastewater treatment plants to improve their profitability by enhancing energy recovery from sludge as well as full utilisation of the existing infrastructure and hence creates a new potential place for alternative treatment of organic industrial waste such as: fat-rich materials or food waste. However, implementation of the solution at wastewater treatment plants is still a big challenge and needs studies including identification of optimal digesting conditions, information about substrate pumping, inhibition thresholds and processing properties. Additionally, due to the characteristics of both co-substrates their introduction to the full-scale digester should be carefully planned due to a potential risk of overloading of the digester. For this reason, a gradual increase of the share of these wastes in the co-digestion mixture is highly recommended, because it will allow for the acclimatization of bacteria as well as prevent overloading. The results of this study show the importance of gradual acclimatization of microorganisms to the changing environmental conditions. It was found that concentration of long chain fatty acids in effluents increased with the reduction of hydraulic retention time, but this phenomenon did not significantly influence the performance and stability of the process probably due to changes hydraulic retention time being gradual. Although for palmitic acid a moderate negative correlation with volatile solids removal was observed.

Anaerobic digestion of sewage sludge with grease trap sludge and municipal solid waste as co-substrates

The feasibility of simultaneous treatment of multiple wastes via co-digestion was studied in semi-continuous mode at mesophilic conditions. The obtained results indicated that sewage sludge, organic fraction of municipal waste (OFMSW) and grease trap sludge (GTS) possess complementary properties that can be combined for successful anaerobic digestion. During the co-digestion period, methane yield and VS removal were significantly higher in comparison to digestion of sewage sludge alone. Addition of GTS to digesters treating sewage sludge resulted in increased VS removal and methane yield up to 13% (from 50 to 56.4) and 52% (from 300 to 456,547m3/Mg VSadd), respectively. While the use of OFMSW as the next co-substrate in the feedstock, can boost methane yield and VS removal up to 82% (300–547m3/Mg VSadd) and approximately 29% (from 50% to 64.7%), respectively. Moreover, the results of the present laboratory study revealed that the addition of co-substrates to the feedstock had a significant influence on biogas composition. During the experiment methane content in biogas ranged from 67% to 69%. While, the concentration of LCFAs was increasing with the gradual increase in the share of co-substrates in the mixtures, wherein only the oleic acid was higher than some inhibition concentrations which have been reported in the literature. However, it did not significantly affect the efficiency of the co-digestion process.

Investigating the possibility of applying an ADM1 based model to a full-scale co-digestion plant

This study investigated the possibility of using a model based on the anaerobic digestion model no. 1 (ADM1) on a full-scale 4000m3 digester in order to understand how such theoretical models can be applied to a real industrial process. The industrial scale digester co-digests the organic fraction of municipal solid waste, grease trap sludge, and ley crop silage with varying feed rates and amounts of volatile solids. A year of process data was collected. Biogas flow, methane content/flow, and ammonia nitrogen were the variables that the model was best at predicting (index of agreement at 0.78, 0.61/0.77, and 0.68, respectively). The model was also used to investigate the effect of increasing the volatile solids (VS) concentration entering the digester. According to simulation results, increasing the influent VS concentration will increase biogas and methane outflow (from 1.5 million Nm3 methane to more than 2 million Nm3 methane), but decrease the amounts of biogas/methane per unit of volatile solids (from about 264Nm3 methane per tonne VS to below 215Nm3 methane per tonne VS).

Pretreatment methods as a means of boosting methane production from sewage sludge and its mixtures with grease trap sludge

The main objective of this study was to determine the applicability of the selected pretreatment methods as a means of intensification of methane production from sewage sludge as well as its mixtures with grease trap sludge. The addition of the fat rich material to the digester treating sewage sludge resulted in an increased methane yield as well as volatile solids (VS) removal of up to 36% (from 134.75 mL/g VS to 182.84 mL/g VS). Furthermore, thermochemical pretreatment of the co-digestion mixture resulted in an approximately 76% higher methane yield as compared to the untreated sewage sludge. The energy balance showed that, for both materials ultrasonic pretreatment and thermochemical pretreatment has an energy self-sufficiency. All of the tested models fit the experimental data with coefficients of determination higher than 0.96.

Enhancement of biogas production from sewage sludge by addition of grease trap sludge

Despite having many benefits, a low degree of volatile solids removal as well as long retention time are the main factors limiting the performance of the anaerobic digestion. Co-digestion of sewage sludge with other organic waste (for example fat rich materials) is one of the few potential ways to enhance the performance of the anaerobic digestion. In this article, the effects of adding fatty rich materials on the performance and stability of semi-continuous anaerobic digestion of sewage sludge were investigated on a 6 l laboratory-scale reactor (working volume equal to 5.5 l). The reactor was operated in a semi-continuous mode with a hydraulic retention time of 10days. The data presented in this paper relate to the period in which the grease trap sludge accounted for 10, 12, 14, 16 and 18% of the mixture on the volatile solids basis.The results clearly indicate that the addition of fat rich materials like grease trap sludge can lead to a satisfactory increase in biogas yield in digester treating sewage sludge. The results showed that co-digestion can enhance the biogas yield by 28–82% compared to anaerobic digestion of sewage sludge alone (control sample). Moreover, the addition of grease trap sludge to digesters resulted in increased volatile solids removal from 44.38% (control sample) to 57.77% (feedstock with 14% addition of grease trap sludge).It was found that the increase of grease trap sludge in the feedstock had a direct impact on the biogas production and methane yield. This proposal has also been confirmed by statistical analysis such as Pearson correlation coefficients and one factor ANOVA. While the long chain fatty acids reduction is not correlated with the gradual increase of fat rich material in the feedstock. The results of the present laboratory study revealed the use of grease trap sludge as a co-substrate is an interesting method for intensification of methane production from sewage sludge.

Anaerobic treatment of sewage sludge and grease trap sludge in continuous co-digestion

In recent years many researchers show a high interest in co-digestion, simultaneous anaerobic decomposition of a homogenous mixture of at least two biodegradable waste. Anaerobic codigestion is reported to offer several benefits over digestion of separate materials, such as increased cost-efficiency, increased biodegradation of the treated materials, as well as increased biogas production. Most often sewage sludge is digested alone while co-digestion with other substrates could be beneficial. In this study, the feasibility of co-digestion sewage sludge and grease trap waste (GTW) from meatprocessing plant was investigated in lab-scale reactor experiment. The research was made on the sewage sludge coming from municipal wastewater treatment plant and grease trap waste coming from meat industry company. Anaerobic co-digestion was studied in semi-continuous experiment at 37oC. Feeding of reactors was performed once a day with hydraulic retention time (HRT) of 10 days. The grease trap waste accounted for 2, 4, 6, 8 and 10 % of the mixture on the volatile solids basis. The mixtures were analyzed for the following parameters: total solids, volatile solids, pH, volatile fatty acids and long chain fatty acids (LCFAs). The control of digestion process was made every day on the basis of the measurement of the biogas production. What is more, there was determined the volatile solid removal as well the biogas yield in order to assess the efficiency of co-digestion process. It was found that co-digestion of sewage sludge and grease trap waste improved both biogas production and methane content. Grease trap waste addition of 10% of feed VS increased the biogas production by 16 % as well as methane concentration (72 % of biogas) compared to the period when reactor was feed only with sewage sludge. Moreover, the addition of GTW to the anaerobic digestion of sewage sludge increased organic matter removal. Although, the significant variations in LCFAs reduction, the biogas production and methane yield increased with higher addition of GTW. The results of the present laboratory study revealed that the use of GTW as a co-substrate is considered to be interesting option for sewage sludge digestion due to increased methane production. However, the feed should be planned carefully with stepwise increase to the desired feed ratio in order to acclimatize the bacteria and to prevent reactor overloading.

Boosting production of methane from sewage sludge by addition of grease trap sludge

Boosting production of methane from sewage sludge by addition of grease trap sludge

A critical review of experimental and predicted methane generation from anaerobic codigestion

Anaerobic digestion is increasingly being considered as a treatment option for an extensive range of waste biomass, due to the potential for energy recovery, in the form of methane production, and lower sludge volumes relative to aerobic treatment processes. Furthermore, when two substrates are codigested (i.e. digested together), added benefits are foreseeable, such as increased methane production and detoxification of toxic compounds via cometabolic degradation pathways. The objectives of this study were to compare experimental and predicted methane production from codigestion literature studies in order to objectively evaluate digester performance. Two predictive methods were used, both assuming methane yields are additive: literature values for digestion of single substrates and a stoichiometric method using model substrates to represent different substrates. Waste sources included in the analysis were primary sewage sludge, waste activated sludge, cow manure, waste paper, grease trap sludge, fat oil and grease and algal sludge. It was found that methane production could approximately be predicted using both methods, with literature methane yields from the same study being the most accurate predictor. One important finding from this study was that the assumption that methane yields are additive is a reasonable one. Furthermore, both predictive methods may be usefully employed as a screening tool to compare methane yields between different types and blends of substrates.

Effect of ultrasound and addition of bacterial product on hydrolysis of by-products from the meat-processing industry

The hydrolysis efficiency of ultrasound (US) and bacterial product (BP; Liquid Certizyme 5™) was studied as a pre-treatment for anaerobic digestion (AD) of four animal by-products (ABP) from the meat-processing industry (digestive tract content + drumsieve waste, dissolved air flotation (DAF) sludge, grease trap sludge and the mixture of these). The change in volatile solids (VS) based hydrolysis parameters was screened using different specific US energy (Es) inputs (1000–14,000 kJ/kg of total solids (TS)) and durations of BP treatment (3–48 h). The Es of 6000–8500 kJ/kg TS increased most extensively the soluble chemical oxygen demand (CODsol) and VS ratio, by 45–290%, and soluble nitrogen (Nsol) by 44–99%. Simultaneously, the average particle size (APS) decreased by 45–75%, from the original value. With BP, the highest increases in CODsol/VS (29–130%), Nsol (35–63%) and decline in APS (40–70%) depended on the raw material (digestive tract content + drumsieve waste: 3 and 24 h; DAF sludge: 3 and 6 h, mixture: 12 and 48 h). BP apparently was more effective at hydrolyzing smaller cellulose particles, while US appeared to enhance the degradation of grease-containing cells.

Increased biogas production at wastewater treatment plants through co-digestion of sewage sludge with grease trap sludge from a meat processing plant

The feasibility of co-digesting grease trap sludge from a meat-processing plant and sewage sludge was studied in batch and reactor experiments at 35 °C. Grease trap sludge had high methane production potential (918 m 3/tVS added), but methane production started slowly. When mixed with sewage sludge, methane production started immediately and the potential increased with increasing grease trap sludge content. Semi-continuous co-digestion of the two materials was found feasible up to grease trap sludge addition of 46% of feed volatile solids (hydraulic retention time 16 d; maximum organic loading rate 3.46 kgVS/m 3 d). Methane production was significantly higher and no effect on the characteristics of the digested material was noticed as compared to digesting sewage sludge alone. At higher grease trap sludge additions (55% and 71% of feed volatile solids), degradation was not complete and methane production either remained the same or decreased.

Effect of pre-treatments on hydrolysis and methane production potentials of by-products from meat-processing industry

In this study, the effect of five pre-treatments (thermal, ultrasound, acid, base and bacterial product) on hydrolysis and methane production potentials of four by-products from meat-processing industry was studied. The bacterial product Liquid Certizyme 5™ increased soluble chemical oxygen demand (CODsol) of digestive tract content and drumsieve waste the most as compared to untreated material (62 and 96%, respectively), while ultrasound was the most effective to increase CODsol with dissolved air flotation (DAF) sludge (88%) and grease trap sludge (188%). In batch experiments, thermal treatment increased methane production potential of drumsieve waste, acid of grease trap sludge and all pre-treatments of DAF sludge. However, with all other pre-treatments, methane production potential was decreased compared to untreated materials, apparently due to inhibition by hydrolysis products and/or possible re-crystallization of some compounds. Methane production potentials from the untreated materials were as follows: digestive tract content 400 ± 50 m 3 CH 4/t volatile solids (VS) added, drumsieve waste 230 ± 20 m 3 CH 4/tVS added, DAF sludge 340 ± 17 m 3 CH 4/tVS added and grease trap sludge 900 ± 44 m 3 CH 4/tVS added.

Co-digestion of grease trap sludge and sewage sludge

Redirection of organic waste, from landfilling or incineration, to biological treatment such as anaerobic digestion is of current interest in the Malmö-Copenhagen region. One type of waste that is expected to be suitable for anaerobic digestion is sludge from grease traps. Separate anaerobic digestion of this waste type and co-digestion with sewage sludge were evaluated. The methane potential was measured in batch laboratory tests, and the methane yield was determined in continuous pilot-scale digestion. Co-digestion of sludge from grease traps and sewage sludge was successfully performed both in laboratory batch and continuous pilot-scale digestion tests. The addition of grease trap sludge to sewage sludge digesters was seen to increase the methane yield of 9-27% when 10-30% of sludge from grease traps (on VS-basis) was added. It was also seen that the grease trap sludge increases the methane yield without increasing the sludge production. Single-substrate digestion of grease trap sludge gave high methane potentials in batch tests, but could not reach stable methane production in continuous digestion.

Anaerobic digestion potential of urban organic waste: a case study in Malmö

A study of existing organic waste types in Malmö, Sweden was performed. The purpose was to gather information about organic waste types in the city to be able to estimate the potential for anaerobic treatment in existing digesters at the wastewater treatment plan (WWTP). The urban organic waste types that could have a significant potential for anaerobic digestion amount to about 50 000 tonnes year(-1) (sludge excluded). Some of the waste types were further evaluated by methane potential tests and continuous pilot-scale digestion. Single-substrate digestion and co-digestion of pre-treated, source-sorted organic fraction of municipal solid waste, wastewater sludge, sludge from grease traps and fruit and vegetable waste were carried out. The experiments showed that codigestion of grease sludge and WWTP sludge was a better way of making use of the methane potential in the grease trap sludge than single-substrate digestion. Another way of increasing the methane production in sludge digesters is to add source-sorted organic fraction of municipal solid waste (SSOFMSW). Adding SSOFMSW (20% of the total volatile solids) gave a 10-15% higher yield than could be expected by comparison with separate digestion of sludge respective SSOFMSW. Co-digestion of sludge and organic waste is beneficial not just for increasing gas production but also for stabilizing the digestion process. This was seen when co-digesting fruit and vegetable waste and sludge. When co-digested with sludge, this waste gave a better result than the separate digestion of fruit and vegetable waste. Considering single-substrate digestion, SSOFMSW is the only waste in the study which makes up a sufficient quantity to be suitable as the base substrate in a full-scale digester that is separated from the sludge digestion. The two types of SSOFMSW tested in the pilot-scale digestion were operated successfully at mesophilic temperature. By adding SSOFMSW, grease trap sludge and fruit and vegetables waste to sludge digesters at the wastewater treatment plant, the yearly energy production from methane could be expected to increase from 24 to 43 GWh.

Bench-scale study of the biodegradation of grease trap sludge with yard trimmings or synthetic food waste via composting

The aim of this study was to evaluate the biodegradation of lipid-rich residues (grease trap sludge) when composted under aerobic conditions, using two different substrates — yard trimmings and synthetic food waste. Addition of lipids seemed to have a very marked effect on the temperature profiles. For the high-rate phase of composting, grease trap sludge was degraded by 39%–51% for the yard trimmings experiments and 10%–27% for the synthetic food waste treatments. For the curing phase, lipids reductions were 9%–26% and 16%–45% for treatments with yard trimmings and synthetic food waste, respectively. For the active phase of composting, the volatile solids biodegradation rate coefficient for the grease trap sludge treatments was 0.009–0.033 d–1, while the average biodegradation rate coefficient for grease trap sludge alone was 0.058 d–1. The yard trimmings treatment with grease trap sludge added at 5% dry solids (ds) resulted in enhanced performance, in terms of temperature profile, rate and extent of biodegradation of solids and lipids, and reduction in wet mass and water content, when compared with the composting of yard trimmings alone. For the synthetic food waste treatments, the addition of grease trap sludge (up to 10% ds) seemed to be inhibitory of the composting process. Key words: lipid residues, compostability, biodegradation rate, biodegradation extent.