My local sanitary utility (county) has required flow attenuation of pool backwash so that it does not overwhelm the pump station. To accomplished this I have used septic tanks or oversized pipe storage with an smaller discharge orifice regulating outflow.

I have routed the flow in order to size the practice to discharge at a required max. rate. In one case, backwash discharged from the filters at 680 gpm. The utility wanted a max. of 100 gpm. It required (3) 1500 gal tanks.

Pool water should be dechlorinated and sent to the storm drain, not sanitary sewer.

some cities do not allow it in storm drains. see below for one example of a large city that does not allow it.

https://www.mesaaz.gov/residents/environmental/dra...

While the City discourages the release of pool water to the City’s storm sewer system, the Arizona Department of Environmental Quality (ADEQ) does provide for these types of discharges under the agency’s De Minimis stormwater permit program (a specialty permit). The property owner or the person conducting these types of discharges is responsible for determining if coverage under this permit applies to their discharge. You must review the conditions required under that program. ADEQ does not charge a fee for these types of permits and no notification to that agency is required. ADEQ De Minimis Stormwater Permit Program: See Part I, Section B, Item 6 on page 6 for discharge conditions established by this agency.

Discharges covered under the ADEQ De Miminis stormwater permit program are allowed to be released to the City’s storm sewer system (including city streets, curb and gutter systems, and alleyways) provided that they are done so in a manner that does not cause a violation of the City’s nuisance code [Title 8, Chapter 6, Section 8-6-3(P)].

Under this alternative, the City allows release into a storm drain if:

It complies with the requirements under the ADEQ De Miminis Stormwater Permit.
It is performed such that the flow of water into or upon a public street or alley does not cause flooding that may impede vehicular or pedestrian traffic, create a hazardous condition to such traffic, cause damage to the public streets or alleys, or cause a condition which constitutes a public nuisance or a threat to the public health and safety. Note: The water must stay within the concrete curb and not spread to the asphalt part of the street.
It is from a source that has not become stagnate and therefore become eutrophic (green algae), polluted, or offensive to the senses.
It does not cause a hazardous or unhealthy condition or facilitate the breeding of insects (mosquito larvae) or cause ponding or oversaturated soils that could cause damage to foundation walls.
If your pool has been subjected to serious contamination (for example, high concentrations of chemical treatment products) or if the water cannot be disposed of under the conditions or other options previously discussed, you may need to hire a contractor to pump you pool into a water truck and dispose of it properly offsite. Check the yellow pages or internet for septic disposal companies for this service.

https://www.mesaaz.gov/residents/environmental/dra...

Most cities allow pool water to be discharged to stormwater if the water is dechlorinate.

Quote (ClemsonVet (Civil/Environmental))

City of Columbia Code requires a standalone interceptor for pool backwash before it hits the Sanitary Sewer. This is a first for me. Anyone else know of an AHJ requiring this? thanks]

The regulation that you are referring to is for a "commercial" pool, not a residential pool. The filter backwash instantaneous flow may be quite large for a commercial pool so this regulation is not that unusual. Water treatment plants with filter backwash needs are required to install backwash holding tanks to equalize the flow.

• https://files.engineering.com/getfile.aspx?folder=b2388372-3954-4593-ad31-d