Decentralized composting has been promoted on a large scale in many Indian cities. But is this the way forward for organic waste management in Urban India? This topic is debatable but I’m now convinced that decentralized composting systems in Bangalore might just add to the environmental damage the city is causing because of improper garbage management.
Let’s look at our composting methodologies:
- Firstly, composting is a science and controlling it artificially is engineering. Unfortunately, most of the plants that I have come across over the past four years are just focusing on converting organic biodegradable waste into a black powder which is assumed to be compost.
- The quality checks are hardly done and the parameters that are measured are C:N ratio (Carbon:Nitrogen ratio), water content and in some cases heavy metal concentrations are checked.
- Odour and leachate control are the most important steps in any urban waste management system. If the composting unit has a bad odour then it implies that it has not been engineered right. A simple mass balance of input waste vs output compost will indicate a huge deficit and hence gaseous emission and leachate needs to be accounted. Composting is a natural process and the decomposition involves liberation of carbon dioxide, water vapour and ammonia but may also generate Hydrogen Sulfide and methane under anaerobic conditions but this needs to be pre-calculated stochiometrically to design the ventilation and odour treatment systems.
- Germination tests and compost stability are hardly measured before it’s application in agricultural fields.
- A large number of composting plants in the city are selling compost to farmers without assessing the quality and this is definitely misguiding the farmers.
Now, let’s understand the composting engineering:
Composting is a natural process of degradation of organic biodegradable waste by aerobic bacteria (requires oxygen for survival) governed by the following equation:
As observed from the above equation, there are few necessary conditions for composting to occur:
- Aeration: Aeration is required for the following functions:
- Transport of excess heat from the plant (this is necessary so that higher temperatures don’t develop and inhibit the process)
- Ventilation – For occupational health of the personnel
- Oxygen demand for composting process as mentioned in the above equation – e.g: Based on established studies, it can be inferred that the amount of oxygen required for biomass degradation is 1.714 kg O2/kg oDM (oDM = organic dry matter) degradation. For 1000 kg wet waste, Oxygen demand = 1.714 kg O2/kg oDM degradation * 300 kg oDM/Ton * 60% oDM degradation (using standard factors). Based on this, the air supply for the composting unit has to be designed.
- Water generation: The water generated in the process needs to be removed (Studies indicate 615g Water/ kg oDM degradation which is often used for calculations).
- Heat generation: Through thermal oxidation, 14kg/g COD (Chemical Oxygen Demand) degradation is released which can be translated to 18.1 MJ / kg oDM.
- Odour Generation: Based on the general equation for composting, it is possible to stochiometrically calculate the concentrations of NH3 and other gases and appropriate biofilters to remove these gases and control the odour needs to be designed.
- Compost Stabilization: It is very important to know when the compost has got stabilized to be used in the fields. There is no general thumb rule for this and hence the respiratory activity needs to be checked. It can be measured using simple apparatus such as Sapromat. This can be measure by either measuring the amount of CO2 released or by the amount of O2 consumed per unit Organic dry matter with a defined time interval. For example, in many EU countries, if a compost has a respiratory rate between 0.5 – 0.3 mg O2/ goDM.h (goDM.h = gram organic matter for one hour), it is considered as raw compost and can be sold for maturation and use. Maturation takes 2-4 weeks and commercial or final compost needs to have <0.2 mg O2/ goDM.h to be used in the fields. This indicates that the compost is stabilized and can be used as manure. Otherwise, it may still be degrading even after application and might not be of any help for the plants in a short interval.
- Compost Hygienization: Since we are dealing with organic waste from municipal solid fraction, there are chances of all kinds of pathogens in the compost such as Enteroviruses, Vibro Cholera, Shigella, etc. Even though the temperatures in compost pile goes as high as 70’C, it may not be sufficient to hygienize the compost. Hence to achieve a safe compost quality, it should be heated above 70’C for 1h or >65’C<70’C for 1 week.
- Application of compost: Apart from the above mentioned parameters, quality check needs to be conducted for every batch of commercial compost to check the C, N, pH, porosity, Na, Mg, Ca, Water content, Electrical conductivity, heavy metals, etc. and the type of compost that can be applied to the field will depend on the crops that are grown. Hence labelling is a very important step for commercial compost.
This is a brief insight into urban decentralized compost engineering and I have not detailed the odour removal and leachate treatment process. It will be easier to implement monitoring systems for composting in few centralized locations rather than several decentralized systems. If the composting plants are not engineered right, then the output must not be used for agricultural production. With respect to composting at home, the quantities of emissions and leachate might be insignificant and the output is generally used in the garden. It may also not be possible for the residents to compost in their premises because of lack of space, lack of expertise and even the will to do it. It’s time we look at Biological treatment systems as Engineering systems and stop assuming the black powder to be a compost!