Purpose and Scope

The Sanitation Districts' policy on Rainwater, Groundwater and other Water Discharges is established under the provisions of Section 305 of the Wastewater Ordinance as amended July 1, 1998. Section 305 specifies that no rainwater, stormwater, groundwater, artesian well water, street drainage, yard drainage, water from yard fountains, ponds or lawn sprays shall be discharged to the Sanitation Districts' sewerage system, except where prior approval has been given by the Chief Engineer and General Manager (Chief Engineer). This document presents guidelines for the implementation of this policy as it applies to rainwater, groundwater and other water discharges mentioned in Section 305.

Rainwater and Stormwater

Note: Rainwater and stormwater are defined terms in the Wastewater Ordinance.

As a general practice, the Sanitation Districts require roofing and/or grading of open areas with exposed drains which discharge to the public sewer, so that all direct rainfall, stormwater and other runoff are conveyed to the storm sewer. This practice protects the Sanitation Districts' sewerage system from excessive hydraulic loads that can be created by stormwater runoff. The Sanitation Districts recognize that there may be situations where roofing and/or grading of exposed areas may be impossible or prohibited by local regulations. Under these conditions, the Sanitation Districts may accept the controlled discharge of rainwater or stormwater to the sewerage system on a case-by-case basis, and only after all other alternatives have been demonstrated to be unfeasible. Applications for discharge of rainwater or stormwater to the sewerage system must include sufficient documentation to demonstrate that no other alternatives are feasible. Alternatives that must be considered include treatment and discharge to the storm sewer, reuse, on-site storage/evaporation and relocation of the processing or treatment areas exposed to rainwater intrusion.

Any rainwater or stormwater accepted into the public sewer from any area larger than 400 square feet is considered to be industrial wastewater and, as such, it must be regulated by a Sanitation Districts' industrial wastewater discharge permit and must comply with all applicable effluent limits. In addition, restrictions may be imposed on the maximum flow rate discharged from a facility during rainfall periods, as well as on the time of day when the discharge of rainwater/stormwater to the sewerage system may occur. Finally, the Sanitation Districts may require rainwater diversion systems, effluent flow monitoring systems, flow restrictors and other devices on a case-by-case basis and as determined necessary by the Chief Engineer. Additional restrictions on the discharge of rainwater to the public sewer may be imposed by the local sewerage agency.

Specific Requirements

1. Roofing or Regrading

Provided that local regulations are satisfied, whenever practical, roofing will be required for all exposed process areas under 4,000 square feet; roofing for exposed areas greater than 4,000 square feet will be determined by the Sanitation Districts on a case-by-case basis. If the roof structure does not include side walls, then the roof's overhang must extend a minimum of 20 percent of the roof's height. In addition, the surrounding area must be graded such that no storm runoff will flow into the roofed area. Finally, all roof drains must be routed to the storm sewer.

Exposed areas that are exempted from the roofing requirement must be properly graded to insure that rainwater and stormwater are prevented from entering into the public sewer. If regrading is required, it should be accomplished by modifying the grade of the property in such a manner that it maximizes the amount of rainwater/stormwater diverted to the storm drain. Existing areas which are exposed to rainwater or stormwater intrusion may be modified to restrict discharge to the sewer by installing permanent berms around all exposed drains, sumps or trenches which are directly connected to the sewerage system. All berms must be built with concrete, brick or other similar waterproof construction material and shall be permanently fixed to the floor with proper bonding (e.g., concrete, mastique, etc.). The berms must not have outlet valves, gates or openings of any kind.

2. Automatic Diversion

If it is determined by the Sanitation Districts that complete segregation of rainwater or stormwater from the sewerage system is not feasible, then only the rainwater/stormwater that flows to the sewer during the first 0.1 inch of rainfall will be allowed as described in the following subsections a, b and c. After the first 0.1 inch of rainfall, excess rainwater or stormwater must be diverted to an approved drainage system by use of an automatic rainwater diversion system. Such systems are generally restricted to industrial facilities with contributing process areas under 10,000 square feet. Larger areas should be regraded to minimize drainage areas, and rainwater or stormwater from non-process areas (e.g., parking lots and roofs) must be routed to the storm sewer system. The automatic diversion system must conform to the County Department of Public Works (DPW) Standard 2043-0, "Rainwater Diversion System", and must be approved by the Sanitation Districts. Other methods of diversion such as automatic valves, manual gates, removable plugs, etc., are prohibited.

Diversion systems are subject to periodic inspections by the local agency and the Sanitation Districts. Companies are responsible for maintaining the systems in proper operating condition to ensure that no excess rainwater or stormwater is discharged to the public sewer. If a company fails to maintain or operate the diversion system to the Sanitation Districts' satisfaction, then the company will be required to roof or properly grade all exposed areas. Furthermore, modifications of existing diversion systems may be required as determined necessary by the Chief Engineer.

As mentioned above, automatic rainwater diversion systems must conform to the County DPW Standard 2043-0 and must include the following elements:

a. Pump Well

The point where the clean rainwater/stormwater is diverted from dry weather industrial wastewater shall be from a pump well as shown on the County DPW Standard 2043-0. During dry weather, the industrial wastewater flows generated in exposed processing areas that discharge into the well must be conveyed by means of a pump to a point upstream of any existing wastewater pretreatment system prior to discharge to the sewerage system. The pump must be selected to ensure sufficient hydraulic retention time in downstream pretreatment systems. The designated pumping rate shall not be exceeded at any time and shall meet any requirements set by the Sanitation Districts and/or the local agency. During rainfall periods, the pump must be automatically deactivated after 0.1 inch of rainfall, and all subsequent rainwater or stormwater that flows into the well must be diverted to the storm drain as explained in subsections b and c below. The diverted water must comply with any water quality standards imposed by the Regional Water Quality Control Board and any other agencies that regulate the storm drainage system. For this reason, industries should make every effort to maintain the exposed area's pavement clean and prevent discharge of industrial wastewater to the pump well during rainfall periods. This will reduce the likelihood of diverting contaminated water to the storm drain.

b. Rain Switch

A device to detect rainfall conforming to the detail shown on the County DPW Standard 2043-0 must be installed in an open area as close as possible to the pump well described above. The rain switch must automatically deactivate the pump whenever it detects 0.1 inch of rainfall so that no excess rainwater/stormwater will be discharged to the public sewer. The switch must not be reset while rain continues to fall. Furthermore, the switch must not be reset earlier than two hours after the cessation of rainfall and until the stormwater flows into the diversion system become negligible. The rain collector must be installed in an area where it will remain directly exposed to rainfall. The collector and switch must also be located in an accessible area, as close as possible to the pump well and at a height not greater than six (6) feet above the adjacent supporting surface (floor, deck or other permanent walkway). This will allow testing of the diversion system safely and without undue difficulty. If the Sanitation Districts determine that it is not feasible to install the collector in an accessible area, then the discharger must provide adequate means for testing the diversion system (i.e., a permanent mechanism to convey water to the collector and a reset mechanism for the switch). A "Y" section with cap for the purpose of testing the rainwater diversion system, as shown in the County DPW Standard 2043-0, will only be allowed when all other means are proven to be impractical.

c. Overflow Line

The diversion of rainwater or stormwater to the storm drain, or other approved point of disposal must be accomplished through the use of a gravity overflow. The overflow line must be located in the pump well or slightly upstream and should be at the same elevation as the pump's effluent line. In the event that a gravity overflow is not feasible due to the surrounding grade of the property, the installation of a second pump to divert all rainwater/stormwater to an approved drainage system may be allowed. This second pump must be automatically activated by the rain switch at the same time that the pump conveying wastewater to the public sewer is deactivated. The rating capacity of the secondary pump must be sufficient to prevent flooding of the surrounding area and possible intrusion of rainwater into the sewerage system. Other methods of diversion such as automatic valves, manual gates, removable plugs, etc. are prohibited.

3. Impoundment of Rainwater in Spill Containment Areas

Except where the discharge of water to the sewer from the first 0.1 inch of rainfall has been approved, the storage and discharge of rainwater to the sewerage system is prohibited. Rainwater that falls directly on tank farms and spill containment areas should be discharged to the storm drain whenever practical. Such discharges shall be in accordance with the requirements set by the Regional Water Quality Control Board or the local agency. If situations arise where this method of disposal is not feasible, the Sanitation Districts may accept the discharge on a case-by-case basis, provided documentation indicates that other alternatives are not feasible. Restrictions may be imposed on wastewater quality, flow rate and time of discharge.

Groundwater

It is the Sanitation Districts' policy to restrict groundwater discharges to the sewerage system. The Sanitation Districts recommend that groundwater either be reused or discharged to the storm drain system. However, in recognition that there may be situations where sewer discharge may be the only viable disposal alternative, the Sanitation Districts may accept the discharge of groundwater on a case-by-case basis, after all other alternatives have been determined to be unfeasible.