2.7.6. Safety
Biogas is primarily composed of methane and carbon dioxide, with traces of ammonia and hydrogen sulphide. Caution should be taken as methane is highly explosive when mixed with air. In addition, because digester gas is heavier than air, it displaces oxygen near the ground, and if hydrogen sulfide is still present, the gas can act as a deadly poison. It is critical that digester systems be designed with adequate venting to avoid these dangerous situations. Exposure to any of these gases may result in ill-health or death, and levels in the biogas may vary widely and cyclically. Carbon dioxide, ammonia and hydrogen sulphide are all toxic gases, and are subject to the regulations as substances hazardous to health [52].
2.7.7. Gas storage
Because biogas is not used at exactly the same rate at which it is produced, it must be stored somewhere. The gas must be efficiently transported from digester to storage tank. Because of the high pressure and low temperature required, it is impractical to liquefy methane for use as a liquid fuel. Instead, the gas can be collected and stored for a period of time until it can be used. The most common means of collecting and storing the gas produced by a digester is with a floating cover-a weighted pontoon that floats on the liquid surface of a collection/storage basin. Skirt plates on the sides of the pontoon extend down into the liquid, thereby creating a seal and preventing the gas from coming into contact with the open atmosphere. High-pressure storage is also possible, but is both more expensive and more dangerous and should be pursued only with the help of a qualified engineer [52].
3. Challenges Associated with Disposal of Hatchery Waste
The cost for an average hatchery to dispose their hatchery waste in Australia is high (Aud$127/tonne and 10.4 tonnes per week). In other countries the cost is greater due to reduced areas available for landfill. The current world population of chickens is approximately 8 billion birds; 90% of these chickens are hatched in commercial hatcheries. The volume of hatchery waste that needs to be disposed of yearly is millions of tonnes. Disposing hatchery waste to land fill causes environmental problems such as releasing methane in the air and possible spread microbial contamination. It is likely that hatcheries in the future will not be permitted to dispose hatchery waste to landfill. Sustainability of these hatcheries is threatened and the challenge is to design a system that converts waste on site to valuable products which can be used on site or sold.
The real challenges for the poultry industry in general is to turn all the waste into economically-valuable outputs using low-cost treatment systems. The huge volume of waste generated by the industry needs to be treated using bioprocesses to produce feed, fertiliser and fuels. These processes need to be applied to the organic waste streams (e.g., poultry manure, hatchery waste) and turn the cost of waste disposal into a source of income, recycle nutrients and reduce pollution This can be achieved by characterising and separating waste, develop products, design systems, and provide risk assessment and quality control. These approaches enable maximum conversion of carbon, nitrogen, phosphorus, and water in waste streams into biofuels and agri-products while at the same time achieving pathogen and odour control.