Matt Hale
The recently updated Anaerobic Digester/Biogas System Operator Guidebook from the U.S. EPA provides a host of useful information about operating a successful anaerobic digestion (AD) plant and maximizing the potential production of both biogas and digestate – the valuable biofertilizer produced by the process. However, while the 83-page document mentions measures to improve the concentration of digestate, it makes no reference to improving the value of digestate.
In the U.S. digestate is often dried for use as animal bedding, an energy-intensive process which does not maximize the environmental benefits of the AD process. By adding organic matter, digestate improves improve soil health and with long term use this can increase the ability of soils to sequester carbon. Using digestate to fertilize soils also reduces the need for synthetic nitrogen, phosphorus, and potassium fertilizers. While the latest AgSTAR operator guidance (https://www.epa.gov/agstar) highlights this fact, it does not cover management or improvement of this valuable resource which can provide additional benefits in terms of both carbon reduction and soil improvement.
HRS Heat Exchangers provides two systems specifically aimed at improving the efficiency of digestate production and maximizing its value in both nutritional and economic terms.
Digestate Concentration
The HRS DCS uses patented technology to remove up to 80% of the water contained in the liquid fraction and concentrates it to 20% dry solids while, at the same time, maximizing the nutrient content, using heat from the AD plant’s CHP engine. Less water also means reduced road and field traffic, bringing further benefits in terms of reduced compaction caused by the application of the digestate to land.
The first part of the DCS process involves heating the liquid digestate in heat exchangers so that minimal additional water and energy is required, as the surplus water from the plant’s CHP engine is used as the heating media. The steam produced from this first cycle is then used as the heating media for the second effect, whereby the process is repeated with further cycles. The DCS is virtually self-sufficient – minimal energy or water is bought in, nothing is wasted, and the surplus energy from the CHP is re-used up to four times.
Another benefit of the DCS is odor control, which helps increase the nutrient content of the digestate. The high temperatures needed to concentrate digestate can cause the release of ammonia, largely responsible for the odors associated with digestate. However, the DCS overcomes this by acid-dosing the digestate with sulfuric acid, thereby decreasing the pH levels. This turns the ammonia into ammonium sulphate, which is not only less odorous, but is also an ideal nutrient.
Digestate Pasteurization
To prevent the spread of potential pathogens which may be present in wastewater, either the feedstock or the digestate should be treated appropriately, with pasteurization being a tried and tested technique across the world.
One of the most energy-, and therefore cost-efficient methods to pasteurize digestate is the HRS Digestate Pasteurization System (DPS), which is based on heat exchangers rather than tanks with heating jackets. Using heat exchangers means that effective digestate pasteurization is possible using surplus heat while allowing additional thermal regeneration levels of up to 60%. This saved heat can then be used for other processes, such as evaporation of the digestate to remove water.
The standard 3-tank DPS provides continuous pasteurization, with one tank being pasteurized while one is filling, and another being emptied. The HRS pasteurizer uses a double tube heat exchanger to heat the digestate to 75 degrees Celsius above the required pasteurization temperature. This allows for variation in the sludge consistency and its incoming temperature, making sure that the digestate is always properly pasteurized. The tanks can also be used individually, for example to allow for routine maintenance.
The DPS and DCS are examples of several systems that HRS produce to improve the efficiency and sustainability of biogas plants and the anaerobic digestion process, with other examples including systems to dry biogas and recover waste heat from exhaust systems. Making biogas plants as efficient as possible will not only increase the environmental benefits they provide but will also improve economic returns for developers and operators, helping to increase the deployment of this vital technology.