Of all the questions a biodigester raises, the one that comes up most often is this: if the waste disappears, where does it go? The short answer is that it goes down the drain, turned into water. The long answer deserves a few paragraphs, because there are nuances it is worth knowing before installing anything.
What exactly is that water?
It is greywater: wastewater with a low organic content, free from suspended solids and contaminants. The aerobic digestion process has gradually broken down the food particles until the microorganisms have been able to consume the remains and convert them into organic water.
This low level of organic matter is the key to ensuring it is safe to discharge. It is not drinking water, nor is it intended to be: it is water suitable for the sanitary sewer system.
Why a sewerage system makes sense
It may seem counterintuitive to pay for equipment only for the end result to end up down the drain. But the logic is sound: if the digested waste has been converted into greywater, the natural thing to do is to use the municipal network to transport it to the treatment facilities, where it is recovered. This infrastructure already exists, has already been paid for, and is designed specifically to treat this type of water.
The alternative — lorries transporting organic waste to a landfill site — consumes fuel, takes up space at the landfill site and generates methane as the waste decomposes there uncontrolled. Compare this with a pipeline.
Use as a fertiliser
Here’s the part that usually comes as a surprise. That water contains dissolved nutrients, mainly nitrogen and phosphorus, which are two of the three basic macronutrients in any fertiliser. This means they can be channelled and utilised rather than being flushed away.
Common uses include watering:
- Green spaces and gardens
- Trees and shrubs
- Industrial crops, such as fibres or biomass
For a hotel with gardens, a university campus or a holiday resort, this can turn water wastage into savings on irrigation costs.
Important notice
Each local authority has its own regulations governing this use, and these must be consulted in advance. It is not a decision that can be taken unilaterally simply because the water «appears» to be suitable. Before considering any reuse for irrigation, it is necessary to check the relevant local authority regulations on discharges and reclaimed water that apply in each case.
The same applies, albeit to a lesser extent, to discharge into the sewerage system: it is advisable to check that the local authority’s discharge regulations cover this type of water.
The other regulation: MARPOL Annex V
In the maritime sector, the framework is different and considerably stricter. Annex V of the MARPOL Convention completely prohibits the discharge of plastics anywhere at sea and drastically restricts the discharge of other types of waste from ships in coastal waters and in so-called «special areas».
For a ship, this makes food waste management a matter of compliance, not cost. The equipment complies with this agreement, and a marine-grade version made from AISI-316 stainless steel is available for use on board.
The environmental assessment
Removing organic matter from landfill has two specific effects: it reduces methane emissions — a greenhouse gas that is much more potent than CO₂ in the short term — and it reduces groundwater pollution caused by leachate.
And within the system itself, the end products of the microbial cycle are reclaimed water, CO₂ and energy. There is no solid waste to manage.
Are you unsure whether your installation meets the discharge requirements? Write to us at the contact page and we’ll go through it with you.
