Stormwater & Greywater Reuse System Design: Engineering Considerations

California’s chronic water scarcity — driven by a combination of growing population, climate-driven drought cycles, and limited freshwater supply infrastructure — has made water efficiency and reuse a central concern of commercial building design throughout the state. Non-potable water reuse systems that capture, treat, and redistribute greywater and harvested stormwater for toilet flushing and irrigation can reduce a commercial building’s potable water consumption by 30 to 40 percent — a contribution that is both environmentally significant and increasingly required or incentivized by California building codes and sustainability certification programs.

At Budlong, our plumbing engineering services team designs greywater reuse, stormwater harvesting, and reclaimed water distribution systems for commercial, institutional, and residential projects throughout California. This guide covers the engineering principles, regulatory framework, treatment requirements, and system design considerations that define professional non-potable water reuse practice in California.

1. Why Water Reuse Matters in California

California relies on a combination of surface water reservoirs, groundwater basins, and imported water — all of which are subject to increasing stress from drought, population growth, and climate change. The state has responded with a combination of mandatory water use restrictions, efficiency standards, and incentives for alternative water supply development — of which non-potable building water reuse is one of the most accessible strategies for individual building owners and developers.

Beyond water conservation, non-potable reuse systems reduce the volume of wastewater discharged to the municipal sewer system, reducing infrastructure loading and associated treatment costs. Buildings pursuing LEED certification, CALGreen Tier 2 compliance, or net-zero water goals rely on non-potable reuse as a primary strategy. MEP plumbing services focused on water conservation at Budlong integrate non-potable reuse analysis as a standard sustainability measure in commercial plumbing design.

2. Greywater vs. Blackwater vs. Reclaimed Water

Understanding the distinctions between water quality categories is essential for designing compliant non-potable water systems in California.

Greywater

Greywater is the portion of building wastewater that does not contain toilet waste. It originates from lavatories, showers, bathtubs, and laundry machines. Greywater has lower pathogen content than blackwater but is not potable — it contains soap, personal care products, skin cells, hair, and low levels of fecal coliform from skin contact. Greywater is suitable for treatment and reuse for non-potable applications after appropriate treatment to meet California’s water quality standards for the intended use.

Blackwater

Blackwater is wastewater from toilets and urinals containing human waste. It has significantly higher pathogen content than greywater and cannot be reused in building plumbing systems without advanced biological and physical treatment to full reclaimed water quality standards. In most commercial building applications, blackwater is discharged to the municipal sewer system rather than treated on-site.

Reclaimed Water (Purple Pipe)

Reclaimed water is municipal wastewater that has been treated by the local water utility to California Title 22 standards and distributed through a separate purple-colored pipe distribution system for non-potable uses. For buildings with access to a municipal recycled water distribution system, using utility-supplied reclaimed water eliminates the need for on-site treatment equipment. The plumbing engineer designs the building’s non-potable distribution system to connect to the utility’s purple pipe rather than to an on-site treatment plant. The future of water conservation in MEP engineering includes expansion of reclaimed water distribution infrastructure throughout California’s urban areas.

3. Non-Potable Water Applications

Identifying the right applications for non-potable reuse water in each specific building is the starting point for system design and sizing. Not all non-potable uses are equal in their demand volume, quality requirements, or engineering complexity.

Toilet and Urinal Flushing

Toilet and urinal flushing is the highest-volume non-potable application in commercial office buildings, schools, and public facilities — typically representing 25 to 35 percent of total building water consumption. It is also the most technically demanding indoor non-potable use because the treated reuse water must be distributed through the building via a dedicated non-potable piping system that parallels the potable cold water system. California requires disinfection with a chlorine residual and turbidity below 2 NTU for toilet flushing applications. This dual-piping requirement adds infrastructure cost but provides the largest water savings of any single non-potable application.

Landscape Irrigation

Landscape irrigation is the highest-volume non-potable application for buildings with significant outdoor planting areas — hotels, campus buildings, and parks. Irrigation from harvested stormwater or treated greywater requires a below-grade cistern, a small pump and pressure system, and distribution piping to the irrigation zones. California allows simpler treatment for irrigation-only reuse than for indoor applications, making irrigation a lower-barrier entry point for non-potable reuse on projects not ready for full dual-piping systems. Sustainable MEP design services at Budlong include non-potable irrigation system design as part of the plumbing engineering scope on appropriate projects.

Cooling Tower Makeup Water

Cooling towers in large commercial buildings evaporate significant volumes of water as their primary heat rejection mechanism. Using treated greywater or harvested stormwater as cooling tower makeup water can substantially reduce potable water consumption in buildings with chilled water plants. The water quality requirements for cooling tower makeup are different from toilet flushing — the primary concerns are scale deposition, biological growth (including Legionella), and corrosion control rather than pathogen limits for human contact. ASHRAE Standard 188 and the facility’s Water Management Plan govern cooling tower water quality management.

4. Greywater Reuse System Components

A commercial greywater reuse system for indoor toilet flushing application consists of several engineered components working in sequence: collection, pre-treatment, treatment, storage, and distribution.

Collection System

Greywater collection begins at the fixture — lavatory sinks, shower drains, and bathtub drains are connected to a separate greywater drain piping system that routes collected water to a treatment system rather than to the sanitary sewer. In new construction, this dual-drain system is straightforward to design and install; in retrofit applications, rerouting existing drain piping to a separate greywater collection system can be significantly more complex and costly. The plumbing engineer must also evaluate the quality and volume of greywater available from the specific sources proposed — greywater from kitchen sinks contains fats, oils, and grease that complicate treatment and is not typically included in commercial greywater collection.

Pre-Treatment

Raw greywater contains hair, soap residue, and particulate matter that must be removed before treatment. Pre-treatment typically consists of a coarse screen or strainer at the collection point, a settling tank or equalization basin that allows heavy solids to settle and provides flow equalization for downstream treatment, and a fine filtration stage. Adequate pre-treatment is critical for protecting the downstream treatment system components from fouling and maintaining consistent treatment performance.

Treatment System

California requires that greywater reused for indoor toilet flushing be treated to: turbidity of 2 NTU or less, no detectable total coliform per 100 mL, and a disinfectant residual (typically chlorine or UV plus chloramine). Packaged treatment systems from manufacturers such as Nexus eWater, Greyter, and others combine filtration (membrane bioreactor, sand filter, or multi-media filter), disinfection (UV plus chlorine), and automated monitoring in a certified system that meets California SWRCB requirements. These packaged systems are the standard approach for commercial applications — custom-designed treatment systems are also acceptable but require independent engineering and regulatory review.

Storage and Distribution

Treated non-potable water is stored in a cistern or storage tank before distribution to toilet and urinal fixtures. The storage volume is sized to provide the buffer needed to match the intermittent supply of greywater to the (also intermittent but differently timed) demand of toilet flushing. A backup potable water connection with an air gap or approved backflow preventer supplements the non-potable supply during periods when greywater generation is insufficient to meet flushing demand. The non-potable distribution system is a dedicated piping network separate from the potable cold water system, color-coded in purple and labeled throughout. MEP drafting services from Budlong include complete non-potable piping documentation on the plumbing drawings.

5. Stormwater Harvesting System Design

Stormwater harvesting collects rainfall from roof surfaces and — sometimes — hardscape areas, routes it through a filtration and treatment system, stores it in a cistern, and distributes it for non-potable uses. It is complementary to greywater reuse and is particularly effective in buildings with large roof areas relative to their water demand.

Catchment Area and Yield Analysis

The primary design input for a stormwater harvesting system is the catchment area — the roof or impervious surface area from which rainfall is collected — and the local rainfall data. A monthly water balance analysis compares the expected stormwater yield (catchment area times monthly rainfall times runoff coefficient) against the anticipated non-potable water demand to determine cistern sizing and the fraction of demand that can be met by stormwater. In California’s Mediterranean climate with dry summers and wet winters, stormwater harvesting systems must be designed with adequate storage to carry water from the wet season to the dry season — or combined with greywater sources that provide year-round supply.

First Flush Diversion

The first flush of rainfall from a roof surface carries the majority of accumulated pollutants — bird droppings, dust, atmospheric deposition, and decomposed organic matter — that have collected since the previous rain event. First flush diverters automatically route the first 0.1 to 0.25 inches of rainfall from each storm event away from the cistern and to the storm drain, improving the quality of water that enters the storage system and reducing treatment requirements. First flush diverter design is specific to the catchment area and local rainfall characteristics.

Cistern Sizing and Overflow Design

Stormwater cisterns must be sized to store the difference between peak wet-season inflow and demand — which can be substantial in California where most rainfall occurs in a few months. Overflow capacity is critical: a fully charged cistern receiving a large storm event must be able to safely discharge excess flow to the storm drainage system without backing up or causing property damage. The cistern overflow connection and the storm drainage system receiving it must both be sized to handle the peak design storm. Plumbing solutions from Budlong include cistern design and overflow engineering as part of the complete stormwater harvesting system scope.

6. Treatment Requirements for California Compliance

California has one of the most developed regulatory frameworks for building water reuse in the United States, but also one of the most complex. Engineers designing non-potable water systems for California commercial buildings must navigate multiple regulatory bodies and code documents.

State Water Resources Control Board (SWRCB)

The California SWRCB’s Division of Drinking Water regulates on-site non-potable water reuse systems through its framework for Alternative Water Sources. Commercial greywater and stormwater reuse systems used for indoor toilet flushing must meet the treatment standards of California Code of Regulations Title 22 for unrestricted urban water reuse — turbidity below 2 NTU, no detectable total coliform, and a chlorine residual of 0.1 mg/L minimum. Local agencies may have additional requirements beyond the state baseline. Sustainable engineering solutions from Budlong incorporate full regulatory compliance analysis as part of the water reuse system design process.

California Plumbing Code Requirements

The California Plumbing Code (CPC) addresses non-potable water systems in Chapter 16 (Reclaimed Water), which establishes requirements for dual-piping system design, cross-connection prevention, labeling, and connection to the potable backup supply. The CPC requires that all non-potable water piping be identified with durable purple color coding and permanent labels reading “Non-Potable Water — Do Not Drink” in English and Spanish at specified intervals and at all valves, outlets, and access panels.

7. Cross-Connection Prevention

Cross-connection prevention is the most safety-critical aspect of non-potable water reuse system design. A cross-connection between the non-potable reuse system and the potable water supply could result in contamination of the building’s drinking water — a serious public health risk.

Physical Separation Requirements

California Plumbing Code requires that potable and non-potable piping systems be physically separated with no direct connections between them. The backup potable water supply to the non-potable storage tank must be provided through an air gap — the most reliable cross-connection prevention device — where the potable supply pipe discharges above the overflow rim of the non-potable storage tank with no physical connection. In no circumstances may a direct connection (even with a backflow preventer) be made between the potable supply and the non-potable distribution system.

Labeling and Identification

All non-potable piping, valves, outlets, and fixtures must be labeled at defined intervals with “Non-Potable Water — Do Not Drink” in both English and Spanish. All non-potable water outlets must be clearly distinguished from potable outlets to prevent accidental consumption. Valve boxes and access panels serving non-potable systems must be labeled with the same warning. These labeling requirements persist for the life of the building and must be maintained by the building operator — the engineer’s design documentation specifies the labeling requirements that contractors must install and operators must maintain.

8. CALGreen and LEED Water Efficiency

Both California’s CALGreen Building Standards Code and the USGBC’s LEED rating system provide frameworks that encourage or require non-potable water reuse in commercial buildings.

CALGreen Non-Potable Water Requirements

CALGreen Division 4.3 (Indoor Water Use) requires commercial buildings of 50,000 square feet or more to include infrastructure for future non-potable water reuse — rough-in piping to toilet and urinal fixture groups with a dedicated stub-out location for future connection to a non-potable supply — even when no active reuse system is initially installed. This future-proofing requirement ensures that the additional cost of dual-piping can be avoided when systems are retrofitted later. CALGreen Tier 2 voluntary compliance includes more aggressive water use reduction requirements that are most effectively achieved through active non-potable reuse. LEED certified building MEP engineering from Budlong incorporates CALGreen compliance documentation as part of the plumbing design package for all California commercial projects.

LEED Water Efficiency Credits

LEED v4.1 awards Water Efficiency credits for indoor water use reduction, outdoor water use reduction, and cooling tower water management. Buildings that supply 100 percent of toilet and urinal flush water from non-potable sources achieve the maximum Indoor Water Use Reduction credits. The LEED water balance calculation must document the source, quality, treatment, and demand for the non-potable water system, requiring the plumbing engineer to prepare a water budget analysis as part of the LEED documentation. Beyond LEED — operational sustainability metrics driven by MEP systems recognizes that water reuse contributes to building performance well beyond what certification credits capture.

9. System Sizing and Supply-Demand Balance

The most important design calculation for a non-potable water reuse system is the supply-demand balance — determining whether the available supply of non-potable water can meet the projected demand over time, accounting for seasonal variability in both supply and demand.

Water Budget Analysis

A monthly water budget analysis tabulates the available supply from each source (greywater volume based on fixture counts and occupancy; stormwater volume based on catchment area and monthly rainfall) and compares it to the projected demand for non-potable water (toilet and urinal flush volumes based on occupancy and fixture frequency of use). The difference between supply and demand in each month determines the required supplemental potable makeup volume and the storage volume needed to carry surplus supply from high-supply to high-demand periods. Plumbing design services at Budlong include water budget analysis as a standard deliverable for non-potable reuse projects.

10. Who Uses Non-Potable Water Reuse Systems?

11. Related Reading

For technical reference, consult the California SWRCB Division of Drinking Water — On-Site Water Reuse resources, the California Plumbing Code Chapter 16 — Reclaimed Water, the USGBC LEED v4.1 Water Efficiency credit library, the ASHRAE Standard 188 for cooling tower water management, and the WateReuse Association technical resources for building water reuse.

12. Frequently Asked Questions