Washington State Department of Ecology

Best Management Practices (BMPs)
For Management and Disposal of Street Wastes


LIQUIDS

Physical and Chemical Characteristics

Physical and Chemical Characteristics

Introduction

The liquid waste left after cleaning catchbasins and other stormwater facilities has two sources: 1) liquid standing in the catchbasin, and 2) the liquid which is introduced to aid in removal of solids.

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.

Physical Characteristics

Standing catchbasin liquid generally has high levels of 'floatables' (e.g. litter, leaves, oil and grease) which often form a scum on top of the liquid. Depending on the interval between storms, this liquid may also be highly turbid or less so if the sediment normally contained in the liquid has had a chance to settle.

Chemical Characteristics

Jacobson (February, 1993) summarized data from five limited studies which examined the constituents of decant liquid as it is discharged from the eductor truck and compared this data with pretreatment standards for Metro, Everett and Lynnwood.


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.


Liquids treatment

While the above 'fresh' decant liquid values often violate wastewate treatment local discharge limits, settleability tests show that minimal treatment can allow decant liquid to pass even restrictive local limits. Limited unpublished data from Metro ( ) report approximately 90% reduction in total metals with 24 hour of undisturbed gravity settling alone. Data in Ecology files attributed to Clint Reed, Kent, WA shows a sample from Midway with 86% of settleable solids settling in 3 hours and 100% in 24 hours. Further testing is needed and individual treatment plants should test their own decant liquids waste on a regular basis, but street waste decant liquid from sites that have passed a visual and historical inspection should meet the most restrictive local limits with 24 hours of undisturbed gravity settling. Overnight settling is more practical and should meet most treatment plant requirements.

Policy Statement

The recommended disposal of street waste liquids is into the sanitary sewer provided the contaminants meet local limits set forth by the local Sewer Authority. Some typical limits include restrictions on heavy metals, oil and grease, and suspended solids. Municipalities with delegated authority may set their own limits based on local conditions. Ecology sets the limits for municipalities without delegated authority. In most cases, the liquid street waste will require treatment prior to discharge into the sanitary sewer in order to meet local limits.

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.


Street Waste Liquid Disposal Recommendations

The following disposal options are only for street waste liquids from sites that have passed a visual and historical inspection. Liquids from eductor (vactor type) cleaning of storm sewer catch basins and detention facilities should be disposed of at a solids/liquids decant station or liquids only decant treatment facility discharging to a permitted wastewater treatment plant. The liquid shall be treated using All Known And Reasonable Technology (AKART) to settle solids and remove other contaminants to meet treatment plant local limits. Appendix D contains specific recommendations for operations at these facilities.

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.

Future Considerations for Street Waste Liquid Disposal

Presently used technology is not adequate to meet water quality criteria for the discharge of street waste liquids to surface water or ground water. This type of discharge may be possible when affordable technology is developed. Water quality discharge limits will have to be met and a discharge permit obtained.

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.


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