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Thermal Hydrolysis Process for Effective and Sustainable Sludge Treatment at Sludge Centers Davide Ferraro1; Ashish Sahu2; Alejandro Jimenez3; Torleiv Næss Ugland4

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Thermal Hydrolysis Process for Effective and Sustainable Sludge Treatment at Sludge Centers Davide Ferraro1; Ashish Sahu2; Alejandro Jimenez3; Torleiv Næss Ugland4 Thermal Hydrolysis Process for effective and sustainable sludge treatment at sludge centers Davide Ferraro1; Ashish Sahu2; Alejandro Jimenez3; Torleiv Næss Ugland4 1 [email protected],Cambi Group AS, Asker (NO) 2 [email protected], Cambi Group AS, Asker (NO) 3 [email protected], Cambi Group AS, Asker (NO) 4 [email protected], HØST Verdien i Avfall AS, Grimstad (NO) Abstract For several small towns in close proximity equipped with wastewater treatment plants (wwtps), a common Sludge Treatment Center (STC) has been found to be a cost effective and a practical solution for sustainable sludge management. Sustainable sludge management is achieved when dewatered sludge from wwtps are treated anaerobically to produce bioenergy. In order to speed up the anaerobic digestion process, the use of thermal hydrolysis process (THP) is used as an effective pretreatment. The aim of this paper is to present the benefits for sludge treatment by applying a THP before anaerobic digestion. Cambi is currently the world leader in Thermal Hydrolysis Process (THP) technology and has more than 60 plants all around the world since 1995. THP has been proven for increasing biogas production, increase the sludge load to existing digesters, and generating a low volume of non- odorous, well dewatered, homogenous and pathogen free biosolids. This in turn reduces the overall carbon footprint of several wwtps and the biosolids can be further used as soil conditioners for agricultural purpose. This has been tried in the UK where nearly 45% of UK sludge is treated by CambiTHP technology. By importing sludges in a single plant (STC) and treating them with THP followed by advanced anaerobic digestion, is an effective way to increase sludge valorisation. The centralised STC solution has been successfully applied by 14 Cambi plants in Europe, Asia and Australia. Spreading high quality biosolids on land is the cheapest and most sustainable way for sludge disposal. A big part of the opposition to agricultural reuse of biosolids has been based upon the appearance and the odour of the product. This study will bring to the attention the experience of applying Cambi biosolids to agriculture and to the characteristics of the cake. 1 1. Introduction Wastewater treatment plant (wwtp) yields two by-products, treated water, which is either reused or discharged to near water bodies based on regulations and second, sludge, which needs further treatment for safe disposal. Sludge a gelatinous mixture of water and particles, which carry pathogens and require further treatment for disposal to the environment. There are several ways of treating sludge, anaerobic digestion, liming, pyrolysis, incineration, and each of these treatments has its own pros and cons in terms of treatment, and overall lifecycle costs (CAPEX and OPEX). Amongst these, anaerobic digestion is the oldest most sustainable way of treating sludge ([1] Ariunbaatar, et al., 2014), which is a biological process and the end products are bioenergy (biogas) and digested sludge. This paper will focus on sustainable treatment of sludge using advanced anaerobic digestion (AAD). There are several unit operations in AAD treatment train, pre-dewatering of sludge from wastewater treatment plant, thermal hydrolysis, anaerobic digestion, final dewatering and biogas handling. All these unit operations in the process line is termed as sludge treatment line (Figure 1). Fig. 1 - Sludge treatment line showing the different unit operations 2. Sludge Treatment Line Sludge from conventional wastewater treatment plants are from primary and secondary treatment, aka, primary sludge (PS) and secondary sludge (SS) / waste activated sludge (from biological treatment using activated sludge process), respectively, or mixed sludge (PS and SS), which are treated in a sludge treatment line ([2] Barber, 2012). In anaerobic digestion, microorganisms biodegrade the sludge in four steps (hydrolysis, acidogenesis, acetogenesis, methanogenesis) and the products obtained are biogas and digested sludge. Hydrolysis is a rate limiting step and there are several technologies that can be implemented to speed this reaction ([1] Ariunbaatar, et al., 2014) and thermal pre-treatment is one of them. This paper will focus on thermal hydrolysis process (THP) as a pre-treatment step before anaerobic digestion. Thermal pre-treatment modifies the sludge flock and improves the degradability of the sludge, by destroying chemical bonds in the cell wall and membrane of the bacteria and in addition carbohydrate, lipids and proteins can become more accessible to biological degradation ([3] Hanjie, 2010). Thus, thermal hydrolysis is one of the important unit operations in advanced anaerobic digestion (AAD) process. The THP consists of three units in this process: Pulper, Reactors, Flash Tank. Raw sludge 2 is dewatered to 16-18 % dry solids (DS) and is continuously fed into the pulper to be mixed and heated by recycled steam. The released process gases are compressed and treated biologically in the anaerobic digesters. Thermal hydrolysis takes place in reactors at 165*C for 20-30 minutes. The sterilized sludge is then passed rapidly to the flash tank, resulting in cell destruction from the pressure drop. The sludge is cooled to the required digestion temperature partly by adding dilution water and partly in heat exchangers. The sludge from THP is followed by anaerobic digestion for biogas production and biogas is further for combined heat and power. Cambi is one of the thermal hydrolysis process (THP) technology provider with 85% global market share with 64 installations in 22 countries. About 34% of the entire UK sludge is currently handled by Cambi. Cambi installations are in large cities such as London, UK; Washington DC, United States; Singapore; Beijing, China and also in smaller locations such as Medina, OH, USA; Hamar, Norway; Vilnius, Lithuania, to name a few. CambiTHP can treat 400 tDS/d to 20 tDS/d. 3. Benefits of AAD While applying thermal hydrolysis process (THP) prior to conventional anaerobic digestion, there are several benefits ([4] Barber, 2016). Studies and full scale installations have revealed on implementing the THP compared to conventional process yields increased digester throughput, upto tripling their capacities, thus reducing the number of digesters in new greenfield projects or optimising the current assets in brownfield projects thus increasing the overall capacity of sludge treatment. The design of THP is also based on modularity, i.e. to increase the treatment plant capacity the number of reactors in THP can be increased. Higher biogas production up to 50% is achieved depending on the feedstock, whether it is primary sludge and/or secondary sludge. One of the primary objectives on implementing the THP is to improve dewaterability of the sludge, which is greatly improved. On dewatering of digested sludge, lower volume of biosolids are produced, with higher dry solids content, which can be easily stored or hauled away. The biosolids have low odour and the end product is guaranteed with pathogen kill to achieve sterilised biosolids. These biosolids can be used as a soil conditioner or organic fertiliser as demonstrated by several utilities worldwide. The implementation of THP results in overall lower lifecycle costs both in terms of CAPEX and OPEX, Capex is reduction in number of digesters to be built, if footprint is the driver, THP can act like a catalyst for sludge treatment thus saving land space. Less sludge needs to be transported, which reduces the overall carbon and ecological footprint. There is lower energy costs as higher bioenergy is produced, which in turn, can be used to run the plant with bioenergy produced and even excess bioenergy can be sold as a product as electricity back to the grid as exhibited by a few ([5] Hackett, 2018). Biosolids, which is odour free and pathogen free have been classified as Class A biosolids by US Environmental Protection Agency (USEPA). This is achieved with AAD. Biosolids of this nature can be land applied or converted into fertiliser. This has been done by large utilities such as DC Water, Washington DC and sold as commercial product known as Bloom, while in the UK, several utilities have used biosolids for land application. Another enterprising company, Høst a leading waste management company convert biosolids into pellets, which are easily transportable and can be stored and exported as a product as fertiliser and sold to farmers around the world. To convert it to fertiliser solar drying technology is implemented. This eliminates landfill and incineration operation for biosolids. 3 4. Sludge Management Sludge can either be treated onsite (at the wwtp facility, which generates the sludge) or offsite, which is decentralised sludge treatment center (STC) and this is possible when the wwtp facility are small and generate small quantities of sludge and is not profitable to have an entire sludge treatment line. Depending on the distances, some small wwtp also send their sludge to a larger wwtp, which has the existing sludge treatment line. In a decentralised STC, sludge from nearby wwtps are dewatered and stored in a slio, where it further undergoes treatment using AAD. Often it happens, there are several different owners of wwtps with independent budgets and governance and one of the major challenges is to gather them under one umbrella to design, build and operate a STC. There are several examples in Europe, especially in UK, where several wwtps are owned by a single operator and they adopt the strategy on decentralised STC to achieve optimisation on assets i.e. to save on equipment costs and operational expenditures as shown in Table 1. A single STC is able to optimise the assets as shown in Table 1. In this example, Plant 1 because of its low sludge volume production cannot implement a THP as its cost prohibitive and this problem can be solved with a common decentralised STC, which will require a reception facility and an external large silo is implemented. The total number of equipment (THP and ancillaries) needed for each plant will also be reduced.
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