Mechanical Biological Treatment plants: The new wave of waste management

I recently had a extended and extremely interesting discussion with an MD of a waste management equipment company in the Western Cape.  As waste management is a high priority and an enormous problem in South Africa at the moment due to budget, space and environmental issues the urgency to explore other options is growing exponentially.

Instead of landfills and incineration, a better option which is more environmentally friendly and a more financially viable solution are Mechanical Biological Treatment (MBT) plants which process MSW (municipal solid waste).  An MBT plant is a waste processing facility that can include waste sorting with the main component being the biological treatment process (wikipedia).  This plant is designed to process a mixture of domestic, commercial and industrial waste.  There has been a big move towards the use of MBT plants in Europe due to legislation which bans the dumping of untreated waste in landfills and the move to compost a larger proportion of MSW .  MBT plants are in operation in almost all European countries with Italy having more than 100 plants and Germany around 45.

So how does an MBT plant work?  Below is a useful process diagram I got from Wikipedia:

Firstly if mixed waste is the input, then the waste stream must be sorted either via mechanical means (typically involves factory style conveyors, industrial magnets, eddy current separators,  trommels,  shredders and other tailor made systems) or by hand to removing recyclables like paper, plastic, glass & metals.

Then the left over organic waste is placed in large anaerobic digestors (wet organic waste) and/or placed in large pipes for bio drying depending on the system configuration of the plant.  The anearobic digestors (decomposition of organic material via micro-organisms in absence of oxygen) produce biogas (methane & CO2) and compost.  The biodrying process uses aerobic microbes to rapidly heat the waste.  The output is a light, dry material which is used for Refuse Derived Fuel (RDF).  All of these outputs can be sold and used in other industries using the concept of #industrialsymbiosis.

The environmental benefits are the following:

• Small fraction of inert residual waste is output
• Reduction of the waste volume to be deposited to at least a half (density > 1.3 t/m³), thus the lifetime of the landfill is at least twice as long as usual
• Utilisation of the leachate in the process
• Landfill gas not problematic as biological component of waste has been stabilised
• Daily covering of landfill not necessary
• Process output can be used for compost, renewable energy (RDF, methane), sold to other industries (CO2 to beverage industry).
• Limiting green house gas emissions such as when waste is incinerated.

There have been a few groups who do not agree with the green credentials put forward by the concept of the MBT plants.  But a combination of a few of these waste management technologies leave us much better off than the situation we are currently in using landfills and incinerators.

In South Africa we only have an MBT plant in Durban which was funded by carbon credits on the futures market.  The plant also has a CHP engine which generates electricity from the heat produced in the aerobic and anaerobic process.  The CHP process is illustrated below:

The possibility of an MBT plant being built in Athlone in Cape Town is a hot topic this year.  The partnership of Wastemart, Fountain Civils, Air Liquide and some key individuals has lead to this plant becoming a reality and a first for the Western Cape.  The inputs and outputs of the plant are certain.  The inputs being the MSW collected in the peninsula which has recyclables removed, so no sorting process is required in the plant.  The ouputs are biogas which will be cleansed by Air Liquide and then compressed and sold, compost which either goes to landfill and/or sold to the agricultural sector and electricity generated via a CHP engine.  The confirmation of the inputs and outputs will help with the raising of the R30 million investment required to build the plant.  Operational costs will be covered by the financial model of the inputs and outputs.

A great deal of process stabilization will be required before and after the plant goes into operation and therefore many opportunities for industrial engineers to get involved.  This plant will in my opinion will solve a great deal of issues currently experienced in the waste industry in the Cape and give key people exposure to this new and growing technology.