Organic waste treatment plant

Until ten years ago, aerobic composting was the only clean technology available to treat organic waste. There were, however, various problems associated with its employment - odours, the need for a large space and the difficulties selling the compost when the original material was not properly selected.

In the mid 1990's a new method became more widespread - biomethanization, i.e. anaerobic fermentation of organic waste. This technology permits the obtention of a mixture of gases - biogas - which can be used to produce electricity, steam for heating or as fuel. This treatment also avoids odours, needs less space and produces a better quality compost.

The combination of both technologies has proved to be best and this is what has been adopted at the Nostián Plant. We shall now take a look at what this process entails.

Waste reception
Organic waste trucks are weighed on entry and again when leaving. They unload in the organic pit and then a crane takes the waste to the hoppers, fitted with biological filters.

Mechanical pretreatment
Bags are opened are non-organic waste that has been included by mistake is separated. Waste is then ground. Continual rotating filters separate three fractions - smaller size waste for anaerobic fermentation, medium size waste for aerobic fermentation and large items, which are rejected, packed and taken to a deposit.

Anaerobic Digestion (fermentation)
After being mixed with water and microorganisms, waste is taken to one of 4 anaerobic digestors (no oxygen) by hydraulic pumps. It stays there for between 28 and 32 days at c.35 º C. Decomposition produces diverse gases (steam, methane), which all together are known as biogas, used for producing electricity.

Dehydration and composting
Organic material from the fermentation process is dehydrated on leaving the digestors, and sent to the composting room. Here is where organic material completes its treatment with aerobic composting for 40 days. It is periodically stirred and fresh air is brought in through ventilators. It is finally taken to the final phase, in which any solids left are eliminated.

Respect for the environment
The final process takes place in closed rooms with dust and odour extraction systems. The inside of these rooms is on a lower level than the outside, which avoids odours escaping.

The whole plant has a series of extra installations for water and air treatment
Biofilters - Air coming out of the plant is purified by a biofilter. This consists of pine cork with a biofilm to eliminate particles and anything that causes odours. Composting is carried out in the presence of ventilators which extract air and take it to the biofilter.

Water purifier - Water from the various processes at the plant is passed through a purifier before being taken to the general city drain system to eliminate potential contamination.