Biomethane efficiency must be maximised

The recent announcement that Sydney’s Malabar Wastewater Treatment Plant will produce biomethane to power up to 24,000 homes is great news, as is the potential for New South Wales to utilise enough biogas to supply 1.4 million homes. However, the secret to maximising the greenhouse gas savings from any renewable energy project is to maximise efficiency at every stage of the process. In the case of anaerobic digestion (AD, the process that generates biogas), this includes maximising the efficient use of the high-quality digestate biofertiliser produced during the AD process, as well as maximising biogas production.

The Malabar Biomethane Injection Project has received considerable coverage in the regional and environmental press, but one thing that none of the published stories mention is the fact that AD creates a valuable organic fertiliser that also offers environmental benefits compared with synthetic fertilisers. However, a key issue the industry is facing is the perception of digestate and how it will be classified to avoid hazardous substance ratings.

By adding organic matter, digestate improves soil health and, with long-term use, this can increase the ability of soils to sequester carbon. Using digestate to fertilise soils also reduces the need for synthetic nitrogen, phosphorus and potassium fertilisers.

HRS Heat Exchangers provides two systems specifically aimed at improving the efficiency of digestate production and maximising its value in 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, maximising 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 odour 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 odours 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 sulfate, which is not only less odorous, but is also an ideal nutrient.

Digestate pasteurisation

To prevent the spread of potential pathogens that may be present in wastewater, either the feedstock or the digestate should be treated appropriately, with pasteurisation being a tried and tested technique across the world.

One of the most energy-efficient, and therefore cost-efficient, methods to pasteurise digestate is the HRS Digestate Pasteurisation System (DPS), which is based on heat exchangers rather than tanks with heating jackets. Using heat exchangers means that effective digestate pasteurisation 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 three-tank DPS provides continuous pasteurisation, with one tank being pasteurised while one is filling and another being emptied. The HRS pasteuriser uses a double-tube heat exchanger to heat the digestate to 75°C above the required pasteurisation temperature. This allows for variation in the sludge consistency and its incoming temperature, making sure that the digestate is always properly pasteurised. The tanks can also be used individually to allow for routine maintenance.

The DPS and DCS are examples of several systems that HRS produces to improve the efficiency and sustainability of biogas plants and the AD 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.

Managing Director of HRS Heat Exchangers Chris Little.

Top image caption: The HRS Digestate Pasteurisation System (DPS) can produce energy savings of 70% compared with other digestate treatment technologies. Credit: HRS Heat Exchangers.