How do you select geosynthetics for biogas containment systems?

By Hermann Ng, Senior Technical Sales Manager, APAC

Biogas production from anaerobic waste stabilization ponds, typically used for treatment of wastewater from farms and industry, is a smart, sustainable, eco-responsible option. A key component of these systems is the geosynthetic material used for the sealing system. Why are polyethylene geomembranes a better choice and how do you select the right material? 

The value of biogas capture is increasingly clear as the energy crises and climate change challenges intensify, and we try to find ways to improve energy resiliency and diversify energy supply. The comparatively low cost (compared to conventional materials) and the ease of enabling biogas capture using geomembranes are driving adoption of biogas solutions. High-density polyethylene (HDPE) geomembranes can be floated on the surface of a containment pond, generally without the need for additional support. Geomembranes also present several other benefits. 

The value of biogas capture 

Waste stabilization ponds built for wastewater treatment reduce organic content and remove pathogens that can harm the environment or pollute freshwater bodies using natural processes The ponds may be used to treat sewage and animal waste, or effluent from industrial, manufacturing or farm processes—e.g., meat, dairy or palm oil processing. Once treated, the effluent may, depending on purity, be used for irrigation or processing, or returned to surface water.   

These ponds may be used singly or linked in a series for successive treatment of effluent. Effective treatment in series often sees effluent treated in a primary anaerobic pond, then a facultative pond and aerobic pond. Anaerobic treatment, the primary stage, reduces organic load by up to 60% by converting organic carbon into methane.  

Traditional open ponding systems release biogas into the atmosphere, causing odor problems and contributing to greenhouse gas (GHG) emissions. These systems are also wasteful. Several studies show significant energy production can result from methane capture. This energy may be used to fuel boilers and turbines, or to produce electricity and heat for domestic or industrial use.  

The more digestible the organic waste matter being processed with anaerobic digestion, the higher the gas yield potential. A good example is this New Zealand study. In it, biogas production from anaerobic stabilization ponds treating piggery and dairy wastewater was measured using floating 269-square-foot (25-m²) HDPE covers on the pond surface. The study found that converting the average volume of methane gas captured from these ponds to electricity would reduce CO₂ equivalent GHG emissions by 6.2 and 0.7 tons (5.6 and 0.6 tonnes) a day, respectively, and generate 1,180 kWh/d and 122 kWh/d. 

Another paper summarizes the experiences in developing Asian countries that installed floating covers on waste lagoons and the benefits they achieved by using polyethylene geomembranes (HDPE or linear low-density polyethylene [LLDPE]) for biogas collection and recovery, energy production, fume and odor control, protection against evaporation, and contamination of groundwater. It discusses the manufacturing quality assurance and quality control program, floating cover design and installation concerns.  

Why geomembranes are a better choice for biogas production 

HDPE geomembranes are highly effective when used as bottom liner systems and as floating membrane covers in biogas systems. With their ability to seal, they keep rainwater and other contaminants out, and keep liquid and gases contained in the pond. By excluding oxygen, they enhance anaerobic digestion activity. They also seal in odors, an unfortunate result of anaerobic activity.

Key characteristics of geomembranes suitable for biogas containment and recovery applications include excellent UV and chemical resistance, greater mechanical resistance against wind uplift, higher impermeability to methane migration, better multiaxial tensile strain, added flexibility (to ease installation, welding quality and speed), and enhanced stress crack resistance. But how are these characteristics achieved, and how do you test for them? 

The geomembranes selected must meet various industry standards and laboratory quality conformance tests, with the demands of the specific application, as specified by the design engineer, tested for. 

Testing for quality

HDPE geomembranes are manufactured from polyethylene resin. Additives include carbon black, heat and UV stabilizers, and antioxidants. High-quality liners will make use of raw materials that meet manufacturers’ standards for delivery of specific, stated levels of resistance to oxidation degradation, stress cracking and heat resistance, and UV stabilization. All of these contribute to the longevity of the material in exposed condition. The use of light or white-colored geomembranes, with their ability to reflect UV light, can deliver additional advantages in specific applications. 

The integrity of liners and covers—at manufacture and at installation—must be ensured. Manufacturers will do electrical spark testing at manufacture to detect pinholes and defects, but electric liner integrity testing must also be done at installation to ensure a secure system. 

Case study: Biogas from palm oil production 

At a palm oil plant in Indonesia, Solmax has helped harness palm oil mill effluent (POME) for renewable energy while protecting the environment from the harmful effects of methane biogas emissions.

POME is an excellent substrate for renewable biogas production—biogas is formed naturally when POME decomposes. A tan-colored Solmax BioCoverPro Liner for biogas containment was selected to line and cover the constructed lagoon. The completed methane capture facility comprised a bottom premium HDPE liner and a premium HDPE cover with the POME contained between these two liners, thus creating an anaerobic digester. 

This premium HDPE liner is backed by extensive research and development to meet the needs of the organic waste industry for covered anaerobic lagoons. A field immersion test in the POME was performed to monitor the liner’s performance and compatibility in the organic waste environment. 

The value for the plant operator is significant. Biogas was captured, distributed, treated and utilized not only as a fuel for two 1,200-kW biogas gensets but also for a biogas burner installed at the existing biomass boiler. This has allowed the operator to reduce dependency on fossil fuel and lower greenhouse gas emissions. 

This article originally appeared on the Solmax blog, www.solmax.com/en/blog.