6.8 Effluent Disposal, Rapid Infiltration Basins, Colorado Water Reuse Law & Regulation 84

Where the treated water goes

WPI's Treatment Process Evaluation outline lists effluent disposal: surface water discharge and rapid infiltration basin (RIB) and solids disposal: landfill as items an operator evaluates and adjusts. In Colorado those choices are shaped by two forces that do not exist in most states: prior appropriation water law, under which effluent is often legally owed to a downstream senior right, and chronic scarcity, which makes reuse valuable.

Surface water discharge

The default pathway. Treated effluent passes through the outfall to a receiving stream under a CDPS permit issued under Regulation 61, with limits derived as described in the Colorado regulatory chapter.

Operational elements the operator manages:

• Outfall structure and diffuser. Kept clear of debris and sediment, and inspected after high flows. Erosion at the outfall is a permit and a public relations issue.
• Compliance sampling point. The permit designates exactly where the sample is taken, and samples taken anywhere else are not compliance data.
• Mixing zone. The permit's water-quality-based limits assume a defined mixing zone in the receiving stream. Operators do not manage the mixing zone, but they should understand that low stream flow shrinks available dilution — which is why summer and drought conditions often coincide with the tightest effective limits.
• Effluent dissolved oxygen and temperature. Many Colorado permits carry minimum DO and maximum temperature limits protective of coldwater aquatic life.
• Whole effluent toxicity (WET) testing. Periodic bioassays on Ceriodaphnia and fathead minnow. A failed WET test triggers a toxicity identification and reduction evaluation, and the most common culprits at domestic plants are ammonia, residual chlorine, metals from an industrial user, and surfactants.

Land treatment and rapid infiltration basins

A rapid infiltration basin (RIB) applies effluent to permeable soil at high rates, allowing it to percolate to groundwater. Additional treatment happens in the soil column through filtration, adsorption, and biological activity. RIBs suit sandy and gravelly soils with adequate depth to groundwater and are common in parts of eastern and southern Colorado.

Operating requirements:

• Loading and resting cycles. Basins are flooded, then rested and allowed to dry. The drying period restores aerobic conditions and oxidizes accumulated organics; continuous flooding drives the basin anaerobic and seals the surface.
• Rotation among cells so each gets its resting period.
• Surface maintenance. Discing, scarifying, or raking the basin floor breaks up the clogging mat of solids and biological growth that forms during flooding. Falling infiltration rate is the symptom that maintenance is overdue.
• Vegetation management to prevent root intrusion and to control mosquitoes.
• Groundwater monitoring wells upgradient and downgradient, with nitrate as the parameter of greatest concern.
• Winter operation in Colorado can be limited by frozen basin surfaces, so storage or an alternate outlet is required.

Other land treatment methods include slow rate application to a crop, and overland flow across gently sloped, vegetated terraces with collection at the toe.

Evaporation, storage, and no-discharge

Some Colorado facilities operate on a total containment or evaporative basis, particularly small and industrial systems in arid areas. Total containment lagoons must be sized on a water balance of inflow plus precipitation minus evaporation and seepage, and they must be lined to prevent groundwater impact. The operator's job is level management — sufficient freeboard for the wet season and for design storms — and monitoring for leakage.

Water reuse in Colorado

Reuse is regulated in Colorado under Regulation 84 (Reclaimed Water Control Regulation), with the reclaimed water treatment facility itself classified under Regulation 100 section 100.6. Two things must be true before a utility can reuse effluent: the water must be legally reusable, and it must meet the quality category for the intended use.

The legal side is genuinely Colorado-specific. Under prior appropriation, water that originated in the stream system and was diverted under a native right generally must be returned to the stream for downstream senior appropriators; it is single-use water. Only certain water may be used to extinction — typically transmountain (transbasin) imported water, nontributary groundwater, and water for which a decreed augmentation plan or reuse decree exists. Denver Water's recycled water system, for example, is built on transmountain and reusable supplies. An operator does not make these determinations, but should understand that "we have effluent, so we can reuse it" is legally wrong in Colorado more often than it is right.

The quality side is category based. Regulation 84 defines categories of use with corresponding treatment and monitoring requirements, running from restricted-access irrigation through unrestricted urban irrigation, industrial uses, and toilet flushing. Higher categories require greater treatment reliability, tighter turbidity and disinfection standards, and more frequent monitoring.

Non-potable distribution requirements that operators enforce daily:

• Complete physical separation from the potable system. There is no acceptable cross connection, and the two systems are never valved together.
• Purple (lavender) pipe, fittings, valve boxes, and signage identifying reclaimed water, per industry convention.
• Backflow protection on the potable side of any facility served by both systems, typically a reduced pressure principle assembly.
• Cross-connection surveys of reuse customers, since an on-site plumbing cross connection between the two systems is the principal risk.
• Public notification and hose bib control to prevent unintended potable consumption.

Potable reuse is emerging in Colorado. Indirect potable reuse discharges highly treated water to an environmental buffer such as a reservoir or aquifer before it is withdrawn and treated again. Direct potable reuse introduces advanced-treated water into a raw water supply or directly to a treatment plant. Both rely on multiple advanced barriers — typically microfiltration or ultrafiltration, reverse osmosis, advanced oxidation with ultraviolet light and hydrogen peroxide, and engineered storage — plus continuous online monitoring with automatic diversion when any critical control point fails.

Solids disposal

The other end of the plant. Options and their controls:

• Land application of biosolids under Regulation 64, with site approval, metals and pathogen requirements, agronomic loading rates, buffer distances, and crop and grazing restrictions for Class B material.
• Landfill. Requires a paint filter liquids test pass, meaning no free liquids, and the landfill's own acceptance criteria. Simple but forgoes the nutrient value.
• Surface disposal and monofills on dedicated sites, with their own Part 503 requirements.
• Incineration, subject to air permitting and rarely used at Colorado's plant sizes.
• Composting with a bulking agent to produce a Class A product.

The operator's recurring duties across all of these are the same: know where the material is going, know the analytical characterization that authorizes it to go there, meter and record the quantity, and keep the manifests. WPI lists maintaining and evaluating operating records and completing required regulatory reports as job tasks, and disposal records are the most commonly audited of them all.