FRP BIOGAS PLANT: For Efficient Kitchen Waste Management

There is worldwide awakening for protection of environment and safe disposal of food/ kitchen waste. In Denmark, it is mandatory that restaurants, public institutions, and other catering centres collect their food waste for recycling. Kitchen waste is available from mess and canteens of industrial establishments, hotels, hostels, religious institutions, etc., where food is prepared for a number of people at a time, in community-type kitchen. All these produce a large quantity of kitchen waste every day, which is generally accumulated near the kitchen. This waste degrades aerobically and, after sometime, emits foul odour and makes the environment near kitchen polluted. Kitchen waste mainly consists of the following:

• Uncooked food waste, such as potato peelings, onion peelings, melon peelings, banana peelings, cabbage, leaves and stem, rotten vegetables, pumpkin peelings, etc.
• Remains of dressed vegetables, such as green pea, green gram, etc.
• Cooked food, such as rice, pieces of chapattis, vegetable waste, tea leaves, etc.

Technology has now been developed for completely prefabricated and portable high-density polyethylene (HDPE), fibre glass reinforced plastic (FRP), rubberized nylon fabric, and RCC-based biogas plants that are being made in factories and assembled on site for installation. The prefabricated biogas plants could be suitable for rural as well as urban areas and would better meet the criteria for judging a technology ready for massscale diffusion.

Methodology

Family-size biogas plant was developed to create better conditions for anaerobic digestion of kitchen waste by separating the three main processes of hydrolysis, acid phase, and methane phase with the help of making compartments inside the digester by providing two partitions having certain heights. These compartments were made of FRP material and their volumes were decided considering the number of days for which the slurry inside the digester was to be kept. The days required for acid phase and methane phase were determined by evaluating laboratory results, where variations of pH along with other parameters were mainly evaluated. The volume of slurry kept for acid phase was further divided into hydrolysis, acid formation, and methane formation phases decided on the basis of past research work. Fresh slurry fed in the digester from inlet pipe is retained in the first compartment for a certain period to create hydrolysis process. Afterwards, the slurry passes into the second compartment by crossing the first partition and is retained there for the acid phase. After a certain time period, the slurry passes into third compartment crossing the second partition. Flow of slurry from one compartment to another also helps to better mix the slurry inside the digester (Figure 1).

The pilot plant was fabricated by FRP and consisted of cylindrical digester with inlet and outlet tanks. The gas dome was made from a special three-layer reinforced fabric namely high-tenacity rubberized nylon fabric coated with hypalon on the outer and neoprene on the inner surface. FRP digester was fabricated in two segments, and they were assembled with a vertical joint at the site. Flanges with holes were provided at the joint and the segments were assembled by bolts, keeping a rubber seal having thickness of 5 mm between the flanges to make the digester leak proof. Digester was fabricated in parts to make its transportation easier and more economical. Construction of FRP digesters is simple and can be done at local level whereas HDPE/LDPE digesters require the fabrication at one central place with the help of costly moulds and then have to be transported to site. Technical specifications of the developed model are given in Table 1.

Results

Biogas volume generated through developed 1 m3 biogas plant was observed daily with the help of dry biogas flow meter. Biogas production varied from 637 to 896 L/day in FRP biogas plant. Average biogas production from FRP plant was 786.05 L/day. The average methane and carbon dioxide content were observed as 57.43 and 39.2 per cent, respectively (Figure 2).

Total solids, total volatile solids, pH, and organic carbon content were observed at the interval of every 20 days. These parameters were observed after stabilization of methanogenic activities in the plant whereby the plant started generating near constant production of the biogas.

It can be observed that the average total solid content of the inlet charge was 9.37 per cent in the mixture of cattle dung and kitchen waste that reduced to 7.5 per cent for the digested slurry. Similarly, the average total volatile solid were calculated as 79.36 per cent of total solids for the inlet charge of the plant, which reduced to 64.35 per cent after digestion.The average Nitrogen, Phosphorous, and Potassium (N,P,K) content in the fresh slurry was observed as 1.19, 0.77, and 0.54 per cent, respectively, whereas the average N,P,K content in digested slurry was observed as 1.41, 0.90, and 0.64 per cent, respectively. Average increase of 16 per cent in N,P,K value in the digested slurry was also observed.

Conclusion

In consideration of the atmospheric pollution and disposable solution of kitchen waste, the developed FRP biogas plant provides feasible solution to public. The feasibility of phase separation and duration of digestion stages was assessed. Family-size FRP biogas plant integrated with phase separation of 1 m3 capacity performed satisfactorily. Average biogas production from FRP plant was 786.05 L/day.

Dr Deepak Sharma, Biogas Development and Training Center, Udaipur, Rajasthan. Email: deepshar@rediffmail.com; Er Kapil Samar, Biogas Development and Training Center, Udaipur, Rajasthan; and Er Amol Shurpatne, Ex Student, Department of Renewable Energy Engineering, College of Technology and Engineering, MPUAT, Udaipur, Rajasthan, India.