Very little data have been collected about the components found in the standing water in catchbasins. However, the liquid is generated during waste collection is of more concern since it contains contaminants from the original standing water plus solids suspended during the waste collection
When a catchbasin is cleaned and the combined solids and standing liquid are vacuumed into the eductor truck, this mixture becomes a 'slurry'. As the truck become full, the slurry off the top of the load is often decanted back into either the storm drain, the sanitary sewer or into a decant station constructed for that purpose in order to make room for more material from other catchbasins. This discussion will cover only the analysis of the characteristics of the slurry decanted from eductor trucks.
King County (Herrera, 1995) analyzed data from 72 decant liquid samples taken from four types of land uses: single family residential, multi-family residential, commercial and industrial. Variability was high and local limits were passed often in the fresh decant.
The street waste liquids may be discharged into a receiving water if the liquids have been tested and the effluent constituents are less than the effluent limits set forth by Washington State's surface and ground water quality standards (Chapters 173-201A and 173-200 WAC). The practice of discharging into the environment is not a preferred disposal alternative because of the probable high treatment, sampling, and analysis costs and environmental risk. However, there may be instances when there is no sanitary sewer within a reasonable travel distance or the local Sewer Authority will not allow the disposal of this waste into their system. Under these circumstances or any other mitigating circumstances, discharge of treated liquids into the stormwater system or receiving water will be necessary.
Disposal facilities for liquids should be made available to all private and public operators that meet disposal facility licensing and site evaluation requirements. Illegal dumping is an unfortunate, but common consequence of public and private operators not having decant receiving locations available. Septage receiving stations have been used to give private operators a liquid disposal site in the Portland area. The relatively small volume (<1,000 gallons) of decant liquid from any one load will provide the treatment plant operator with a additional margin of safety.
Decanting of liquids back into the storm sewer catch basin or detention facilities that they were taken from is allowed as part of the maintenance of the system and only if no other practical means for disposal are available, the facility is remote from surface waters, the liquids will not leave the site for at least 24 hours, and All Known And Reasonable Technology (AKART) is used before discharge.
Decanting of liquids and solids at specially constructed wet ponds or evaporation basins would be useful at isolated sites and appears to be technologically feasible, but has not been done. Water quality discharge limits will again have to be met and a discharge permit obtained.
The discharge of eductor truck liquids that have not been contaminated with catchbasin solids (from picking up water to get to a flooded catch basin, for example) is treated the same as catchbasin cleaning decant liquid. It does not make sense that pumping liquids directly to the next catch basin is allowed while carrying them in a clean truck is not. Preventative cleaning of catch basins would solve this problem in many but not all cases.
 © 2005 World Sweeper
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