Belcher, Hannah

Hannah Belcher holds an MSc in Biochemical Engineering and is currently an Operations Manager for Severn Trent;. Responsible for the performance and...

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About Belcher, Hannah

Hannah Belcher holds an MSc in Biochemical Engineering and is currently an Operations Manager for Severn Trent;. Responsible for the performance and operation of 6 sites including a THP that is capable of generating over 200 MWh per day. Her work with Orege UK was first presented in November 2023 at the European Biosolids and Bioresources Conference. She is currently working on the Net Zero site commissioning and implementation.

Belcher, Hannah

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Description: Increase in Biogas Production and Other Benefits Resulting from SLG-F Advanced...

Increase in Biogas Production and Other Benefits Resulting from SLG-F Advanced Sludge Thickening and Conditioning Prior to Anaerobic Digestion at Worcester STW, UK

Abstract

1. Context The recent focus for improving Anaerobic Digestion (AD) has been on the acceleration or preparation of sludges to improve gas yields through the process. In this context, Severn Trent developed its interest in Orege's application, SLG-F advanced sludge thickening / sludge conditioning technology for boosting AD performance. Severn Trent and Orege agreed to a 4-month, 2-stage trial over the spring and summer of 2023 at Worcester (Severn Trent Water) STW to better understand the application of the technology at a commercial scale in the UK. This paper presents the experience of STW including installation & commissioning, process performance data across the whole wastewater treatment plant and the increase in biogas production and CHP output recorded during the trial period. The innovative sludge thickening / conditioning package plant incorporates the SLGTM (Solid-Liquid-Gas) process developed by Orege and has already been installed as plug-and play containerised systems in the UK either to replace existing sludge thickening assets or where no mechanical sludge thickening was carried out. 2. Introduction Anaerobic digestion is a process commonly used on sewage treatment works throughout water and sewage companies (WASCs) in the UK. Many of these digesters were built in the 1960's. Since then, population increase means that many of these AD plants are now at maximum design capacity. There is a reluctance to build additional digesters to deal with future growth as technology progresses, therefore new solutions must be trialled. Worcester STW is a prime example of this; the site was expected to reach its built capacity by 2028, with new permits introduced by the Environment Agency additional dosing needed to be included producing additional sludge, this meant the digesters would be at full loading by 2025. Worcester is a sewage treatment facility just outside the city of Worcester; it serves a population circa 100k. It is also a reception centre for sludge imports from satellite sites in the local area. All imports are accepted into three reception tanks and manually transferred to a pre-digestion tank. All primary sludges are thickened in the primary sediment tanks (PSTs). Surplus activated sludge (SAS) is thickened using two gravity belt thickeners. All sludges are pumped into the mixed, pre-digestion tank, before being fed to the digesters. The digestion for the site consists of two conventional digesters, each 2900m3 in volume. They are operated following the site HACCP (Hazard Analysis and Critical Control Points) plan in order to ensure compliant cake for land application. The two CCPs for this HACCP plan are a minimum temperature of 34oC and a maximum feed volume of 161m3 per digester per day. As per Severn Trent standard, the digesters are mixed using biogas. 3. Methodology All onsite thickening during both phases of the trial was carried out by Orege SLG-Flosep technology (see figure6). SLGTM (Solid Liquid Gas) is a technology that has been developed and patented by Orege and is a sludge pre-treatment system which, using only air and polyelectrolyte, dramatically increases solid and water separation. The patented process is cost-effective and simple to operate. The Flosep is an optimised liquid/solids separation device that takes advantage of the unique characteristics of SLG conditioned sludge. Four thickening units were used onsite. Two of these units thickening a mixture of primary sludge and imported sludge. The other two units replaced the two belt thickeners that were being used for SAS thickening. All thickened sludge was pumped from these units into the pre-digestion tank before being fed to the digesters. Phase 1 consisted of analysing the impact of solely running SLG-F thickening units. Phase 2 consisted of phase 1, with the additional use of SLG-AD on the thickened SAS. SLG-AD consisted of an additional tank that recirculated the thickened SAS whilst injecting compressed air through two SLG units. The SLG conditioned sludge was then pumped into the pre-digestion tank. 4. Results & Discussion The impact of increased dry solids (%DS) on the digesters was analysed daily over the duration of phase 1. The analysis of the digesters can be separated into different blocks: digester control, biogas production and yield, and digester health. All results discussed are from phase 1 only. 4.1. Digester Control 4.1.1 TDS/day and OLR The %DS of the digester feed increased from 4.9% to 5.6% during phase 1. In the HACCP plan for Worcester, it states that the maximum feed volume is 161m3/day, the decision was made to continue to feed this volume despite the change in dry solids. This meant that the tonnes of dry solids (TDS) being fed increased. This took the TDS from 7.1TDS/d to 7.8TDS. (see figure 1) In parallel of the TDS, the Organic Loading rate was calculated and studied. The OLR baseline was 2.46kgVS/m3/d, this average increased to 2.78kgVS/m3/d. This increase is also due to the maintained volume feeding the digester at a higher DS%. Moreover, on some days the OLR was consistently above 3. With conventional digestion, the standard maximum OLR is 3kgVS/m3/d, as it was shown that a higher OLR can cause stress to the methanogens and therefore volatile solids destruction struggles to be done effectively. 4.1.2. Hydraulic Retention Time (HRT) As mentioned, the maximum feed per digester at Worcester is 161m3, which is an HRT of 16 days. As the volume remained the same the HRT was kept consistent at 16 days. 4.1.4 Temperature and other parameters A temperature increase in both digesters was observed over phase 1. Temperature went from 36.1oC to 38.3oC; or 6% increase (see figure 3). This is partially due to the increase in %DS. Less heat energy is required to heat sludge compared to water. This increase in temperature was also sustained, and the site was able to turn off the diesel-powered boiler and solely rely on heat from the CHP. Other parameters such as pH, alkalinity and Volatile Fatty Acids were monitored and showed a healthy digester. 4.2. Biogas Production and Yield 4.2.1. Biogas Production An increase in biogas produced was observed from 4760m3/day to 6586m3/day, corresponding to +38%. In addition, when stable and consistent feed volumes of 161m3 per day were achieved the biogas production increased to 7048m3/day over a two-week period - an increase of over 48% compared to the baseline (See figure 4). This increase was due to two reasons. The first, is the increase in OLR. The second, can be believed to be due to the use of SLG-F for thickening. The biogas yield increased from 338.8m3biogas/TDS to 404.3m3biogas/TDS. If the increase in biogas produced was solely down to the increase in OLR, the amount of biogas produced per TDS would have stayed approximately the same. This suggests that there has been a change in the sludge characteristics. Sludge could have changed due to the sludge ratio. However, even with an increase in sludge volumes, the mass ratio still stayed around 60:40, Primary:SAS. 4.2.2. Methane Content There was no difference in the percentage of methane content in the biogas. It can be difficult to obtain an increase in methane production as it depends on the colonies of methanogens. 4.2.3. CHP Output During Phase 1 the CHP output increased by over 34% from a baseline of 9.9MWh to 13.3MWh. The MWh/TDS produced increased from 0.73MWh/TDS to 0.92MWh/TDS during phase one. The data used for phase 1 however, does not include any flare data so this figure is likely to increase. 5.0 Conclusions Phase one of the trial was very successful. The use of SLG-F for all thickening onsite increased the overall digester feed %DS by 0.6%. This increase in the dry solids allowed greater utilisation of the digester capacity, but also a greater overall plant capacity and the daily import for the site was almost doubled. The OLR increase and the thickened sludge characteristic change correlated with an increase in biogas production of 38%. Another benefit was the reduction in the need to run the diesel boiler; this had a significant cost saving. 6.0 Phase 2 The aim of phase two was to determine if the addition of SLG-AD, which consisted of recirculating the thickened SAS to two SLG units, would increase the biogas yield further. Unfortunately, in the early stages of this phase, both digesters experienced blocking and spills due to RAG. These blockages meant that the digester feeds continuously stop-started. The data collected from this phase of the trial was inconclusive. However, the biogas yield showed a positive difference compared to baseline (see table 1). 6.1. Phase 2 at Monkmoor STW Phase two of the trial is due to be replicated at Monkmoor STW, Shrewsbury later this year. Key words:, Thickening, Biosolids, Innovative, Anaerobic Digestion, Biogas, Methane, Sludge, CHP, Package Plant, Severn Trent, Orege, SLG, SLG-F.

This paper was presented at the WEF Residuals and Biosolids Conference, June 18-21, 2024.

SpeakerBelcher, Hannah

Presentation time

09:30:00

10:00:00

Session time

08:30:00

10:00:00

SessionThickening

Session number10

Session locationOklahoma City Convention Center, Oklahoma City, Oklahoma

TopicAnaerobic Digestion, Biogas Production, Biosolids

Author(s)

Belcher, Hannah

Author(s)H. Belcher1, C. Justier2

Author affiliation(s)Severn Trent Water1; Orege North America 2;

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Jun 2024

DOI10.2175/193864718825159447

Volume / Issue

Content sourceResiduals and Biosolids Conference

Word count24

Base text size -

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