Alternate: Thermal Hydrolysis for All: Options for Small and Medium Size Plants

Anaerobic digestion of particulate substrates like municipal sludge is a multi-step biological process that begins with hydrolysis: the breaking down of larger macromolecules into small molecules amenable to fermentation. As seen in Figure 1, hydrolysis is the rate-limiting step for sludge digestion and this recognition has resulted in the creation of many different technologies over the last 20 years to try to speed up the hydrolysis process. Development of these technologies has generally been pursued to maximize the efficiency of existing anaerobic digestion assets including increasing volatile solids (VS) destruction and biogas generation. Of these technologies, thermal hydrolysis has had staying power and become the 'de facto' sludge pretreatment alternative most commonly proposed for digestion process intensification. As its name implies, the thermal hydrolysis process (THP) enhances hydrolysis by treating wastewater solids with high temperature. An established THP technology applied in North America, thermo-pressure, utilizes a combination of high temperature and high pressure to facilitate the breakdown of sludge solids. The typical advantages of this THP include intensification of digester capacity typically two-fold, i.e., doubling the allowed VS loading rate, reduced sludge viscosity facilitating digestion at much higher total solids (TS) concentrations, higher VS destruction and biogas yield, improved dewatering resulting in higher cake dry solids content and less wet biosolids cake product transportation, as well as a high-quality end-product for beneficial reuse. The principal disadvantages of this THP typically cited include operational complexity, specialized staff technical knowledge and skills to operate and maintain the high temperature and pressure systems as well as the increased ammonia recycle loads associated with higher VS destruction. The discussion above assumes that THP is used as pretreatment for anaerobic digestion, but as shown in Figure 2, there are at least four different process configurations for using THP. It may be used for hydrolysis of both primary and waste activated sludge (WAS) labeled Pre-Digestion All in the figure. To maximize the benefit per dollar spent on capital as well as operating and maintenance cost, THP may be used only to improve hydrolysis of the more difficult to digest WAS, labeled as Pre-Digestion WAS-only. THP may also be used to treat digestate the benefits of doing so include achieving high cake solids concentrations in subsequent dewatering and producing a sterile product. The filtrate or centrate, containing the bulk of the solubilized organics, may be recycled to the digester to increase biogas production and reduce the organic load recycled to the liquid treatment plant. This is labeled Post-Digestion. One may also consider intermediate THP, that is feed digestate to THP prior to further digestion, as another strategy to reduce the cost of implementing THP. Finally, one may feed dewatered sludge to the THP so that THP is used to improve solids quality prior to disposal, regardless of the level of treatment prior to THP, labeled Post-Dewatering. The application of THP to date has been limited to larger plant sizes as shown in Figure 3. Most of the THP facilities in North America are at large facilities such as Blue Plains (Washington, DC), TRA CRWS (Dallas, TX), Bonnybrook (Calgary) and others. In the last several years smaller package THP units have been introduced to the North American market for medium size plants, of which there are two installations at plants rated 60 ML/d or 15 mgd (Franklin, TN) and 113 ML/d or 30 mgd (Pontiac, MI), which are still relatively large the majority of plants throughout North America are much smaller. An alternative to thermo-pressure is thermo-alkaline hydrolysis which uses elevated temperature and pH for hydrolysis and is a technology that can be readily applied at small and medium size plants. This type of THP addresses the complexity and high temperature/pressure issues by using lower processing temperatures and ambient pressure with the tradeoff of adding an alkali chemical. It can be used in either a Pre-Digestion or a Post-Dewatering configuration. In other words, thermo-alkaline is also an option for facilities that do not have any form of sludge stabilization. There are 13 full-scale facilities of this technology operating in North America since 2006. Most of those facilities use the Post-Dewatering configuration to create Class A quality high-solids liquid fertilizers. Operation of thermal-alkaline hydrolysis in one of the Pre-Digestion configurations is a more recent direction for this technology in North America. Pre-Digestion has been more commonly utilized in Europe. Lystek is operating at several small plants rated 5 to 15 ML/d (1.3 4.0 mgd) and several medium size plants rated 27 to 50 ML/d (7 14 mgd); some of these recycle a portion of the thermally hydrolyzed product back to anaerobic digestion for additional solids destruction and biogas generation. The technology is also used at the regional 400 ton/day Organics Material Recovery Facility at the Fairfield-Suisan Sewer District plant in Fairfield, CA. Lystek is currently developing the Pre-Digestion configuration. Pondus has one installation in North America at a plant rated 106 ML/d (28 mgd) in a Pre-Digestion configuration, with more than ten similar systems installed in Europe at facilities as small as 38 ML/d or 10 mgd. THP for small and medium size plants is arguably an even better value proposition than at large plants because smaller facilities often dispose of biosolids in liquid form and may have inefficient, or even no, digestion and/or dewatering processes. THP can improve product quality and reduce viscosity to improve disposal of liquid biosolids, allowing direct subsurface application on arable land, so that there is no need for the farmer to store the product prior to land application. Table 1 compares the thermal-alkaline hydrolysis (from both Lystek and Pondus suppliers) with more widely known thermo-pressure hydrolysis processes (from Cambi, and other suppliers). The difference in complexity and operating conditions are included. Note than Pondus is the only technology that treats solids at typical thickened solids concentrations as commonly used for anaerobic digester feed. The Lystek system uses potassium hydroxide to improve the fertilizer value of the product. The thermo-alkaline processes can be used to provide substrate for biological nutrient removal systems. Thermo-pressure and the Pondus system are typically used to improve anaerobic digestion performance, in other words to increase VS destruction so that biogas production is increased and the mass of residual biosolids is reduced. The Lystek technology, by comparison, has historically been used to convert residual biosolids into a marketable fertilizer product and only more recently starting to be applied as pretreatment to improve anaerobic digestion. The goals of THP may be different at different size plants. At large, urban plants the focus may be to maximize energy recovery and minimize the mass of residuals needing disposal, while reducing the use of chemicals where possible. Thermo-pressure THP is well suited to this application. Large plants are also better equipped to meet the complexity and specialized staffing requirements of thermo-pressure THP. The increased recycle ammonia load may be addressed by implementing another specialized sidestream treatment technology also feasible at a large plant. At smaller and especially rural plants, the ability of some thermo-alkaline THP to convert partially or unstabilized sludge into a marketable fertilizer product would be prized. Improving hydrolysis is of limited value to a plant that does not implement anaerobic digestion or where the biogas from anaerobic digestion is flared because the flow is too low to make cogen or other energy recovery options viable. However, with higher prices of natural gas, sale price premiums for renewable natural gas, and small cogen units like microturbines, implementing anaerobic digestion with thermo-alkaline hydrolysis at small plants is becoming increasingly feasible. Implementation of THP at smaller facilities also opens the opportunity for public-private-partnerships to reduce risks and securing private equity, or public-public-partnerships to gain economy of scale and overcome challenges due to skilled labor and part-time dewatering operations. Thermo-alkaline THP is less complex and operates at lower temperature and pressure conditions than thermo-pressure THP systems. In addition, Post-Dewatering configurations allow unstabilized solids to be treated by THP, so that the technology can be applied at plants without anaerobic, or any, digestion. That has allowed thermo-alkaline THP technology and its benefits to be implemented at plants as small as 5 ML/d or 1.3 mgd.