Additional load of using kitchen sink waste disposal units on municipal wastewater treatment

Question

Organic (in the generally-from-an-organism sense of the word) waste can be difficult to dispose of from dwellings with no outdoor space for composting, and (in New Zealand, at least) it is not collected separately from general rubbish without incurring a substantial additional cost.

This means apartment dwellers etc are left with the following choices for disposing of their organic waste:

1. Mix it in with the general non-recyclable waste
2. Pay extra for commercial collection
3. Use an under-sink waste disposal unit to pulverise the waste and dispose of it through the wastewater system, mixed with water

I've commonly heard references to the problems caused by mixing it with general waste, which include leachate from landfills, high methane emissions, and odour problems prior to collection.

To better understand the environmental impact of people's choices, I'm wondering if measurement has been done of the additional cost incurred by wastewater treatment plants in treatment of the additional waste in the system from the third option above.

Both an economic cost and an environmental cost (such as a carbon footprint), would be useful to have.

It's worth mentioning that other options may also be a possibility in some cases, such as a local community compost facility. This might be something we can all aspire to have in future.

Answer 1

Life cycle assessment has been done on this! This report is by InSinkErator, so they obviously have some commercial interest. Here's the summary, which (to me) seems sound: http://www.insinkerator.com/en-us/Documents/Disposer/ISE-Life-Cycle-Summary.pdf

I will search out the original study report, which will define more of the assumptions they're making and what is being included and excluded from the model.

Answer 2

Consider this answer a draft till I found some numbers.

Additional waste in wastewater will generate energy costs in two ways:

• additional aeration: Biomass is degraded by pumping air into the wastewater, thus providing aerobic bacteria with the oxygen to digest the waste. The pumping of air is energy intensive
• additional sludge: The aerobic digestion process creates sludge, consisting of non-digested matter and the aerobic bacteria. This has to be treated (de-watered, applied to land, evtl. burnt, ...) - actually a huge part of the costs of any WWTP is the sludge treatment.

To get a rough figure, one would do this calculation (this needs some data from the wastes):
VS content of wastes X conversion factor VS to BOD X energy cost per BOD
Ash content of wastes X some factor for the sludge yield X energy cost per mass of sludge
This should account for most (energy costs). To fully understand the impact of the kitchen wastes one needs to know the downstream processes of the WWTP - for example, is the sludge burned, applied as fertilizer or used in some other way.

Note the following: If the WWTP has an anaerobic degistion before the aerobic stage, the kitchen wastes wil actually generate energy per methanization.

On the other hand, landfills are at best just a postponement of the wastes for later generations. I think it's hard to quantify the carbon footprint of a landfill because we don't know yet what will happen to the site in 10 or 50 or 100 years.