Greywater Reuse Systems

A number of different configurations are given. The GRS Reedbed–Dripper System enables treated water to be passed onto garden beds via dripper irrigation (e.g. Uniram or Tiran 17 purple pipe), while the GRS Reedbed-Trench system contains a subsurface piped trench system to accommodate overflow water from the reedbed.

The design of the reedbed is such that optimal conditions for bacterial growth is present and the nature of the substrate will permit coarse filtering of the greywater. Stone or gravel has a high porosity and thus permits a greater permeability of water. The retention time in each tank is between five to eight days. This will provide a retention time of up to fifteen days for the system, which is usually ample for greywater treatment in these types of constructed wetlands.

Each reedbed will permit subsurface horizontal flow of the wastewater, and the system is designed to prevent ponding - which will decrease the risk of unpleasant odours and mosquitoes and other pests.

Sizes
The size of the reed beds vary with the number of householders. Generally a figure of 4 m2 per person is used for the calculations for a complete greywater reuse system. For example, dispersal of all greywater from a 3 bedroom (4 person) house would require two reedbeds, each with a surface area of 8 m2 (total = 16 m2).

The reedbed is most often one to two metres wide and up to five metres long. This prevents short-circuiting from occurring and allows the maximum contact of the wastewater with the substrate and plants as the wastewater moves throughout the bed.

The size of the reedbeds for a partial greywater reuse system varies with the number of people and the expected volume of reuse water. Table 1 shows the reedbed sizes for different applications. As an example, the reuse of the laundry and bathroom water in a four bedroom home would require two reedbeds, each having a surface area of 9 m2, or any combination of reedbeds having a total surface area of 18 m2. Base figures for partial greywater reuse systems are laundry 1.5 m2/person, bathroom 2.0 m2/per person and combination 3.5 m2/person.

 Table 1. Total surface area requirements (m²) for different greywater reuse applications.

Liners
Liners can be either fibreglass, medium to high density polyethylene or PVC (all membranes to be UV-stabilised).

Typically, liners are 2 – 3 mm thick, but thicker liners can be used.

If a flexible membrane is used, geotextile cloth is used for both below and above the liner to protect the liner from possible damage.
Reedbed Structure

Each bed has a layer of coarse stone (10 to 20 mm), usually 600 mm deep, which is contained in a trench lined with an impervious membrane A layer of bentonite clay could also be used in place of the plastic sheeting. The reedbed system relies on both evaporation from the soil and transpiration by the plants, and is thus called an evapotranspiration system.

Figure 1 shows the arrangement of two reedbeds (placed in series). Wastewater from the bottom of the first reedbed is directed into the top of the second bed. This figure is a summary of the whole system, with a settling tank and an Inspection Opening in each bed to permit visual inspection of the water level and to enable pipe cleaning if required.

Figure 1.  Cross-section of complete GRS reedbed system.

Figure 2.  Experimental reedbed built to determine the effectiveness of several plants in greywater treatment.

GRS Reedbed-Dripper System
Treated water finally passes into a pump chamber, which has nominally at least 250 L capacity but can be 500 to 600 L if required to maintain a larger irrigation field. A safety level warning device is fitted to sound/alarm should the submersible float switch pump fail for any reason.

The dripper system uses purple pipe with drippers typically at 300 mm spacing. Treated effluent first passes through a standard irrigation disk filter before entering the dripper pipe. The irrigation field can be set up to use a KRain rotary valve or other make of sequential valve to send water to different parts of the garden in a cycle.
GRS Reedbed-Trench System

Any overflow from the reedbeds can also be directed into a subsurface piped trench, such as the GRS Standard Piped Trench or the GRS Mini Piped Trench. The required length of piped trench can be ascertained from a GRS consultant.