Environmental Impact
Kitchen waste disposal units increase the load of organic carbon that reaches water treatment plant, which in turn increases the consumption of oxygen. Metcalf & Eddy quantified this impact as 0.04 pounds of biochemical oxygen demand per person per day where disposers are used. An Australian study that compared in-sink food processing to composting alternatives via a life cycle assessment found that while the in-sink disposal performed well with respect to climate change, acidification, and energy usage, it did contribute to eutrophication and toxicity potentials
This may result in higher costs for energy needed to supply oxygen in secondary operations. However, if the waste water treatment is finely controlled, the organic carbon in the food may help to keep the bacterial decomposition running, as carbon may be deficient in that process. This increased carbon serves as an inexpensive and continuous source of carbon necessary for biologic nutrient removal.
One result is larger amounts of solid residue from the waste-water treatment process. According to a study at the East Bay Municipal Utility District’s wastewater treatment plant funded by the EPA, food waste produces three times the biogas as compared to municipal sewage sludge. The value of the biogas produced from anaerobic digestion of food waste appears to exceed the cost of processing the food waste and disposing of the residual biosolids (based on a LAX Airport proposal to divert 8,000 tons/year of bulk food waste.)
In a study at the Hyperion (Los Angeles) sewage treatment works, disposer use showed minimal to no impact on the total biosolids byproduct from sewage treatment and similarly minimal impact on handling processes as the high volatile solids destruction (VSD) from food waste yield a minimum amount of solids in residue.
Energy usage is not high; typically 500 - 1500 W of power is used, comparable to an electric iron, but only for a very short time, totaling approximately 3-4 kWh of electricity per household per year. Daily water usage varies, but is typically one gallon of water per person per day, comparable to an additional toilet flush. One survey of these food processing units found a slight increase in household water use.
Read more about this topic: Garbage Disposal Unit
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