Combining Thermal Hydrolysis with Drying and Downstream Thermal Processes to Optimize Energy Recovery from Sewage Sludge

Combining thermal hydrolysis with drying and downstream thermal processes to optimize energy recovery from sewage sludge W. P. F. Barber*, and P. Christy*, * Cambi Inc., 279 Great Valley Parkway, Malvern, 19355, PA, USA (E-mail: [email protected]; [email protected]) Abstract Thermal hydrolysis of sewage sludge prior to subsequent anaerobic digestion is a well-established process with nearly 90 facilities worldwide. Cambi, the market leader, has two thirds of the installations ranging in size from 11 – 450 t DS/d. The majority of plants are combined with land use of the biosolids as a low carbon fertilizer due to the biosolids meeting Class A standard under the US EPA 503 regulations. In addition, the dewatered biosolids are friable, have low odor and high dry solids which make it a desirable product. However, there are instances where land application is limited due to regulations or topography, or clients have existing thermal infrastructure and want to maximize its use. The aim of this paper is to demonstrate how combining thermal hydrolysis with digestion prior to thermal systems such as drying and incineration can be beneficial and highlights full-scale case studies to quantify the influence in the field. Keywords Anaerobic Digestion; thermal hydrolysis; drying; incineration INTRODUCTION Thermal processing of sludge, such as drying or incineration, can play an important role within the development of a water company’s strategic plan for biosolids management. In addition to regulatory requirements which may exist, there may be insufficient local land available (due to topographical limitation for example), or the sludge may contain other materials limiting its use. Furthermore, water utilities may already have existing infrastructure based on thermal treatment and prolonging their use may provide an economic long-term option compared to installing new infrastructure. Additionally, some dried biosolids materials have had sustained use as a fertilizer product (Clark, 1930). In spite of the advantages of thermal processing – primarily sludge reduction, there are several disadvantages and these have led to a trend in Europe where many drying and incineration systems have been, or are being decommissioned. Table 1 highlights the advantages and disadvantages of both drying and incineration. Table 1. Advantages and Disadvantages of sludge drying and incineration Advantages Disadvantages Drying Proven at full-scale for sewage sludge High capital and operating costs compared with other methods of water extraction High volume reduction compared with Needs auxiliary fuel such as natural (or bio) gas dewatering Meets Class A as defined by US EPA 503 Potentially complex operation regulations Storage and handling of product may be easier Sludge goes through a “sticky phase” which causes than sludge cake (especially if pelletised) handling problems in the drier and may require recycling of dry sludge. However, this may be overcome by use of indirect driers Long storage times possible Very sensitive to fluctuations in load. If this happens 2797 Advantages Disadvantages either dry solids output is compromised or sludge throughput must be reduced Larger range of disposal options than biosolids Issue with fibres cake Increases heating value of sludge prior to thermal Chemicals (such as ferric) may have adverse effects on destruction the thermodynamic properties of the dried sludge. This has caused explosions in stored dried material. Material can be used as a fuel With direct driers, production of gas which may require further treatment. Use of digestion or heat recovery can partially Critically reliant on gas prices and may become offset the energy demand of drying uneconomical with slight fluctuation in gas price Rewetted dried pellets may be odorous, especially for raw dried sludge Not a final disposal option, will compete with high quality Class A dewatered cake for agricultural outlets Expensive to turn into fuel so third party can exploit value of at negligible or no cost Can be high carbon footprint in absence of on-site renewable energy availability. High dry solids sludge can be dusty which reduces its desirability in certain reuse markets Incineration Highest Volume Reduction Very low on waste hierarchy Potential for energy recovery Requires complex gas abatement technology Ultimate disposal option Generates carbon dioxide which could incur future taxes Fly ash may be recycled, and nutrients extracted Negative publicity (especially on current sites) Not from viewed by public as acceptable solution. Environmental groups calling for incinerator tax schemes to be implemented Standard technology Removes phosphorous (a non-renewable resource) from ecosystem Reduces reliance on landfill Produces a number of wastes, most of which are classified as hazardous requiring special treatment options High capital and operating cost compared to alternatives THEORY Energy requirements of drying One of the main issues with drying is the energy required to evaporate water. As well as dryer type, the quantity of energy required by a sludge dryer is critically dependent on the temperature and type of sludge which is being dried. The type of sludge influences both thermodynamic properties and dewaterability (i.e. water content) of the sludge being dried. Typically, the greater the fraction of waste activated sludge, the higher the water content within the material to be dried and the higher the energy demand for the dryer. From theory, assuming an ambient temperature of 20°C, the energy required to evaporate one metric tonne of water, is 2.34 MMBTU. In addition to the water, the dry matter in the sludge has to be 2798 heated. At the same ambient temperature, based on the specific heat capacity for dried sludge (Ref), heating one tonne dry matter of sludge would require an additional 0.10 MMBTU. These numbers assume no heat loss and need to be adjusted to account for dryer heat losses and potential heat recovery both of which are dependent on type of dryer. From the literature (refs), dryers typically require 800 kWhr to 1100 kWhr of energy for each metric tonne water evaporated. In imperial units this can be in the range of 3.4 MMBTU/t water evaporated, although operational data – as we will see – is typically higher. To put this number into context, this is orders of magnitude higher than the energy required to remove a ton of water by dewatering and then transporting it. The US Department of Energy (Ref) provides data on the energy use of transport with respect to hauling weight (ref). From that report, a heavy truck consumes 3357 BTU energy to move one ton one mile, therefore on an energy basis it is possible to move a ton of water over 1000 miles for the same energy expenditure as evaporating it in a dryer. Using rail, it is possible to transport a ton of water over 10,000 miles to the other side of the world. Fortunately, at a wastewater treatment works, some of the energy required for sludge drying can be recovered by use of waste heat or by using biogas produced from anaerobic digestion. In addition to producing biogas, a further benefit of anaerobic digestion is reducing the quantity of sludge requiring drying. Thermal hydrolysis and sludge drying As with anaerobic digestion, thermal hydrolysis can increase capacity of existing drying plants, or significantly reduce the size of new-build facilities. In order to demonstrate the influence of anaerobic digestion with and without thermal hydrolysis pre-treatment, a hypothetical example based on drying a dewatered sludge cake containing 100 dry tons/day of either primary, waste-activated (WAS) or mixed (60-40 ratio primary-WAS) sludge to 90% dry solids from ambient temperature is used. The example is based on expected dewaterability (Higgins et al., 2015; Phothilangka et al., 2008; Everett, 1972) and digestion performance (Van Dijk & de Man 2010; Stuckey & McCarty, 1984; Haug et al., 1978) Figure 1, shows the impact of digestion with and without thermal hydrolysis pretreatment on water evaporation requirements (size) for the hypothetical dryer. 2799 Figure 1. Influence of digestion and digestion with thermal hydrolysis pre-treatment on water evaporation of drying. Key: () primary sludge; () mixed sludge (60:40 primary:WAS); () waste activated sludge. What is immediately noticeable is the influence of sludge type on the dryer size. Regardless of presence of digestion or pre-treatment, a dryer processing waste activated sludge will be approximately double the size of one processing an equivalent amount of primary sludge. This is due to the poor dewaterability of waste activated sludge which is limited by presence of extracellular polymeric material (refs), compared to primary sludge. Depending on sludge type, anaerobic digestion reduces dryer size by between 30% (for pure WAS) to 40% (for pure primary). This reduction is due to conversion of sludge to biogas upstream. When thermal hydrolysis is added, a larger quantity of sludge is converted to biogas, but importantly regarding drying, the dewaterability is significantly improved. This improvement is typically 10% points (Barber, 2016). The combination of further enhanced volatile solids destruction and improved dewaterability reduces the drying by 65 – 70% in size compared to the raw sludge dryer. Even compared to the option with digestion, drying requirements are reduced by half when thermal hydrolysis is introduced. Figure 2, summarizes these findings in a normalized way for the mixed sludge, based on the size of a raw sludge dryer being the baseline. 2800 Figure 2. Influence of digestion with and without thermal hydrolysis pre-treatment on size of sludge drying Figure 3 shows how the energy requirements of drying can be offset by the production of biogas from anaerobic digestion. In this example using the data for mixed sludge, the dryer requires a little under 700 MMBtu/d of energy to run. Anaerobic digestion reduces the energy demand to approximately 580 MMBtu/d, whilst the addition of thermal hydrolysis lowers it further to 330 MMBtu/d.

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