How Thermal Hydrolysis Influences The Economics Of Biosolids Management

Thermal hydrolysis continues to gain in popularity around the world. There are over 120 facilities worldwide and several across the United States. In North America, operating plants range from approximately 10 to over 400 dry tons processed daily. DC Water's Blue Plains facility, the first in North America, has been operational since 2014. Whilst many studies have looked at the influence of the technology on biosolids management, virtually none of have been specifically focused on the detailed financial impacts. Furthermore, when costs are presented within biosolids master plans, they are inconsistent across projects and can have a wide variance. Financial drivers play a major role in decision making with respect to biosolids processing and generation of master plans. However, whilst important, the financial aspects of a project are multifaceted and involve numerous other influencing factors. These include, amongst others: basic cost of capital equipment (varies with supplier); impact and use of existing infrastructure (how much can be reused or repurposed); requirement for future investment (for example, will new digesters be required); contingency; liquor treatment; delays (in construction); standards/requirements, use of unions, incentives etc. From experience, financial impacts are also geographically dependent making global benchmarking of projects challenging. For example, costs are consistently lower in the UK, where it accounts for 50% of all the sludge produced, than they are in the US. Figure 1 shows the influence of plant size on normalized capital costs in the UK. Although a key component in many projects, it only contributes between 5 and 50% of the overall cost with the emphasis on the lower end. Figure 1 demonstrates economies with scale, becoming more cost effective as projects increase in size. However, modularization has reduced costs recently, making thermal hydrolysis cost effective at smaller sizes than previously possible. Thermal hydrolysis reduces the costs of sludge processing centers. It does this by increasing capacity of digestion, drying, incineration and composting. It is well documented that capital costs at Washington DC's Blue Plains facility fundamentally reduced due to thermal hydrolysis. However, depending on what infrastructure is on site, there additional investment in auxiliary equipment may be required. New research on reducing retention times in digestion by using thermal hydrolysis suggests that capital costs can decrease further. With current knowledge, the number of anaerobic digesters at Blue Plains could reduce further from 4 to only 2. saving a further $80M. Furthermore, construction influences overall costs, with delays adding to but also deferring from making cost savings. Additionally, plans for future expansion can be deferred, minimized, or eliminated altogether, as thermal hydrolysis can be installed as part of a step-wise program. Additional, due to the higher loading rates possible, capacity on existing digestion plants, can be used for the digestion of organic wastes with further influence on economics. From an operational viewpoint, more biogas is produced which can provide revenue, and a combination of enhanced anaerobic digestion performance with superior dewaterability result in significant reductions in biosolids cake resulting in cost savings. At Blue Plains, production of higher value biosolids has resulted in revenue from sales, and since installing thermal hydrolysis, DC Water are saving approximately $20M annually of which $15M is due to reduced biosolids production. Ongoing work is looking at reducing costs further by selling portions of the biosolids cake. However, sludge must be thickened prior to processing, energy is required for thermal hydrolysis to meet the heating needs, better digestion performance releases more nutrients which then need to be treated with chemicals and electricity. More infrastructure needs additional maintenance, and additional biogas needs to be processed further prior to use.
The aim of this paper is to look at the capital costs of thermal hydrolysis as well as the influence it has on both capital and operating costs of biosolids processing based on plant data and modelling. Several sensitivity analyses will be presented based on different configurations of the technology comparing it to a facility where thermal hydrolysis is absent, in order to show where the main influencing parameters are. These will also be compared to data available from various studies and actual cost savings. An example of the model is given in Figure 2. Which shows the typical operational breakdown of standard mesophilic anaerobic digestion compared with that preceded by thermal hydrolysis. Figure 2 is based on all sludge costs from thickening to biosolids recycling (to land, in this case) inclusive of the treatment of the liquors released by anaerobic digestion. For ease of comparison, costs are combined, i.e. electricity refers to electricity required by thickening, digestion mixing, pumping, downstream dewatering and liquor treatment. The figure clearly shows the influence of cake application costs, assumed here to be $45/wet ton. At that cost, cake recycling accounts for nearly 2/3rds of the entire biosolids processing costs. This reduces to approximately 40% with thermal hydrolysis due to the introduction of new parameters. After costs related to cake production, costs for liquor treatment are next most influential and can be up to a third of the whole operating cost. Although a great deal of focus is placed on the costs of maintenance and energy required for steam generation, these are much less significant than would otherwise be expected. Although more processing is being done with thermal hydrolysis, the running costs are between 70 and 80% of those without pretreatment. They remain lower due to the large reduction in cake production. When combining these reduced costs with increased revenue from biogas, the costs of running thermal hydrolysis with digestion are between 40 and 60% of those when there is no pretreatment.
This paper will look at adding additional processing steps, such as drying, (and partial drying with thermal hydrolysis and digestion – Figure 3) and look at the specific impacts of adjusting the price of utilities such as polymer and gas. The impact of additional thickening prior to thermal hydrolysis to reduce heat requirement will be investigated to see if there is a financial benefit to thermally hydrolyze higher dry solids sludge in order to reduce steam consumption. The paper will show the impact of entire biosolids processing on overall costs. For example, full-scale experience in Europe has shown that intentionally making equipment underperform may result in operational cost savings. In addition, the influence of different uses of biogas, for steam generation, electricity production or biomethane will be determined. The influence of using natural gas or biogas for drying will also be reviewed by means of examples. In combination from various full-scale facilities, a number of normalized costs as $/wet ton will be presented along with suggestions on how to minimize the ownership costs of thermal hydrolysis.