Selection of an appropriate biogas storage system makes a significant contribution to the efficiency and safety of a biogas plant. A biogas storage system also compensates fluctuations in the production and consumption of biogas as well as temperature-related changes in volume. There are two broad categories of biogas sto
rage systems: Internal Biogas Storage Tanks are integrated into the anaerobic digester while External Biogas Holders are separated from the digester forming autonomous components of a biogas plant.
The simplest and least expensive storage systems for on-site applications and intermediate storage of biogas are low-pressure systems. Floating gas holders on the digester form a low-pressure storage option for biogas systems. These systems typically operate at pressures below 2 psi. Floating gas holders can be made of steel, fiberglass, or a flexible fabric. A separate tank may be used with a floating gas holder for the storage of the digestate and also storage of the raw biogas. A major advantage of a digester with an integral gas storage component is the reduced capital cost of the system.
The least expensive and most trouble-free gas holder is the flexible inflatable fabric top, as it does not react with the H2S in the biogas and is integral to the digester. These types of covers are often used with plug-flow and complete-mix digesters. Flexible membrane materials commonly used for these gas holders include high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low density polyethylene (LLDPE), and chlorosulfonated polyethylene covered polyester. Thicknesses for cover materials typically vary from 0.5 to 2.5 millimeters.
Plastic consumption has grown at a tremendous rate over the past two decades as plastics now play an important role in all aspects of modern lifestyle. Plastics are used in the manufacture of numerous products such as protective packaging, lightweight and safety components in cars, mobile phones, insulation materials in buildings, domestic appliances, furniture items, medical devices etc. Plastics are used because they are easy and cheap to make and they can last a long time. Disposal of plastic waste has emerged as an important environmental challenge and its recycling is facing roadblocks due to their non-degradable nature. Because plastic does not decompose biologically, the amount of plastic waste in our surroundings is steadily increasing. More than 90% of the articles found on the sea beaches contain plastic. Plastic waste is often the most objectionable kind of litter and will be visible for months in landfill sites without degrading.
Recycling and reuse of plastics is gaining importance as a sustainable method for plastic waste disposal. Unfortunately, plastic is much more difficult to recycle than materials like glass, aluminum or paper. A common problem with recycling plastics is that plastics are often made up of more than one kind of polymer or there may be some sort of fibre added to the plastic (a composite). Plastic polymers require greater processing to be recycled as each type melts at different temperatures and has different properties, so careful separation is necessary. Moreover, most plastics are not highly compatible with one another. Apart from familiar applications like recycling bottles and industrial packaging film, there are also new developments e.g. the Recovinyl initiative of the PVC industry (covering pipes, window frames, roofing membranes and flooring).
Polyethlene terephthalate (PET) and high density polyethylene (HDPE) bottles have proven to have high recyclability and are taken by most curbside and drop-off recycling programs. The growth of bottle recycling has been facilitated by the development of processing technologies that increase product purities and reduce operational costs. Recycled PET and HDPE have many uses and well-established markets.
In contrast, recycling of polyvinyl chloride (PVC) bottles and other materials is limited. A major problem in the recycling of PVC is the high chlorine content in raw PVC (around 56 percent of the polymer’s weight) and the high levels of hazardous additives added to the polymer to achieve the desired material quality. As a result, PVC requires separation from other plastics before mechanical recycling.