Waste Disposal
MSD’s largest solid waste stream consists of sludge cake, all of which is incinerated when the incinerators are properly functioning. Hauling sludge cake to a landfill is only used as a backup measure. Other solid waste streams generated by the wastewater treatment process are handled as follows:
- Screenings, grit, and scum. As wastewater enters each treatment plant, a screening process removes particles such as paper towels and other non-biodegradable materials, as well as grit (such as sand or gravel). Further in the treatment process, scum accumulates at the top of the clarifiers (large settling tanks), which MSD skims off. MSD collects screenings, grit, and scum from each facility and transports it to the Mill Creek treatment plant. MSD contracts with a waste hauler to collect the accumulated screenings and grit and remove them to a disposal facility. Scum is reprocessed at the Mill Creek plant before it is also hauled away for disposal.
- Ash. This material is the result of incineration; it is put into lagoons for storage and then hauled by a waste management company to a disposal facility, where it is used as landfill cover. Lagoons are located at the Mill Creek and Little Miami plants, in proximity to the incinerators that produce the ash. Mill Creek’s lagoon is emptied several times a year; however, the Little Miami lagoon can go as long as 10 years without having to be emptied.
The figure below summarizes the quantities of MSD’s wastewater treatment-related solid waste disposed of in 2009.
2009 Waste Disposed in Solid Waste Landfills
Solid Waste Type | Quantity Disposed in Landfills in 2009 |
---|---|
Sludge cake | 1,144 dry tons |
Screenings, grit, and scum | 7,354 tons |
Ash | 13,672 tons |
Chemical Composition of Ash
Ash is the residual material from sludge incineration, during which all volatile compounds are burned off. Because MSD stores ash in lagoons prior to land filling, testing the concentration of hazardous or carcinogenic constituents is important for both the safety of the crews and the community. The ash is composed of sand, with some metal residuals. Keeping the ash wet allows for the metals to settle at the bottom of the pond. In 2009, the average concentration of heavy metals in the ash lagoons is shown below.
2009 Concentration of Regulated Contaminants in Ash
Regulated Contaminant | 2009 Average Concentration in Ash (mg/L) | Regulated Limit (mg/L) |
---|---|---|
Arsenic | <0.05 | 5 |
Barium | <0.05 | 100 |
Cadmium | <0.05 | 1 |
Chromium | <0.05 | 5 |
Lead | <0.05 | 5 |
Mercury | <0.0020 | 0.2 |