Treating Household Greywater with Constructed Reed Beds

Constructed wetlands, often called reed beds due to the dominance of hardy emergent plants like Phragmites and Typha, offer an elegant low-energy approach for treating household greywater. Rather than sending laundry and shower water to a conventional septic system, a reed bed channels the flow through a shallow basin filled with gravel and plant roots. As the water meanders through this living filter, suspended solids settle, nutrients are taken up by plants and microbial communities break down organic compounds. The size of the bed determines how long the water remains within the system, a parameter known as hydraulic retention time. Sufficient retention allows the biological processes to complete, producing a clarified effluent suitable for subsurface irrigation or safe discharge. This calculator helps homeowners and designers estimate the required surface area of a reed bed based on daily greywater generation, desired retention time and bed depth.

The calculation begins with estimating the daily volume of greywater produced by the occupants. Typical domestic usage ranges from 40 to 90 liters per person per day depending on water conservation habits and appliance efficiency. The total daily volume is the product of the number of users and the per-person generation rate. To ensure complete treatment, designers specify a hydraulic retention time, often between two and five days for subsurface flow systems. Multiplying the daily volume by the retention time gives the total volume that must be contained within the bed. Dividing this volume by the bed depth yields the necessary surface area. The relationship is compact enough to capture in MathML:

where A is the surface area in square meters, N is the number of users, q is greywater generation per person per day in liters, t is the retention time in days and d is the bed depth in meters. Because volume is expressed in liters, dividing by depth in meters and by 1000 converts the result to square meters. The calculator performs these unit conversions automatically and also assumes a rectangular bed with a length twice its width for output dimensions.

As an example, consider a family of four producing 50 liters per person per day. With a desired retention time of three days and a bed depth of 0.6 meters, the total greywater volume is 600 liters. Dividing by depth and converting units results in a required surface area of about one square meter. Using the length-to-width ratio of 2:1, the bed would be roughly 1.4 meters long and 0.7 meters wide. In reality, designers often build a slightly larger bed to account for future growth, variations in water use and the benefits of lower hydraulic loading, which tends to improve treatment efficiency and plant health.

The table below shows example sizes for various household scenarios, assuming a bed depth of 0.6 meters and a 2:1 length-to-width ratio. These values highlight how retention time and greywater volume influence the footprint.

Plants play a central role in the performance of a reed bed. Their roots leak oxygen into the gravel, creating microenvironments where aerobic microbes thrive. These microorganisms metabolize organic matter and pathogens. Periodic harvesting of plant biomass removes nutrients from the system, preventing eutrophication. Designers often choose a mix of species to provide resilience and year-round function. Depth influences plant selection: shallow beds suit grasses and sedges, while deeper beds support reeds and cattails. The calculator's depth field allows users to experiment with different plant communities and hydraulic considerations.

Infiltration rate of the surrounding soil also matters. Some systems discharge the treated water directly to a drain field, while others recirculate it for irrigation. A bed built atop poorly draining clay may need an underdrain or a liner to prevent waterlogging. Conversely, sandy soils may allow some water to percolate before full treatment, necessitating a thicker layer of gravel or a waterproof liner. These site-specific factors should be evaluated alongside the calculator's output to finalize dimensions.

Regular maintenance ensures longevity. Sediment traps or grease filters upstream of the bed prevent clogging. Inspecting inlet and outlet pipes, trimming vegetation and occasionally raking the gravel surface can keep flow uniform. Because the calculator estimates the active treatment area, designers should allocate additional space around the bed for access paths and any pre-treatment components. Properly sized and maintained, a reed bed can provide decades of reliable greywater treatment with minimal energy input and an aesthetically pleasing appearance that blends into the landscape.

Beyond the practical aspects, reed beds offer ecological co-benefits. They provide habitat for insects, amphibians and birds, increase biodiversity and create educational opportunities about natural water cycles. Integrating greywater treatment into a garden or permaculture system closes nutrient loops and reduces demand on freshwater resources. The calculator is a starting point for realizing these benefits, translating household water habits into physical dimensions that guide construction and planting.