BHW Message: Managing Greywater

Greywater is wastewater from sinks, laundry washing, showers and bathtubs. It may contain bacteria and pathogens, oil and grease, and food wastes, and if allowed to pond near homes, can cause nuisances and health hazards. Greywater is properly managed in four ways:

1. It can be piped to septic tanks and treated with the blackwater from toilets;
2. It can be piped to a dedicated greywater treatment system and then disposed of in soil-based leaching trenches;
3. It can be piped to a dedicated greywater treatment tank and then reused to water plants or flush toilets; or
4. It can be piped to city sewers or approved stormwater/wastewater drainage canals.

In each case, greywater must be piped away from the home where it can not pose a threat to people, be a breeding site for mosquitoes and flies, or cause odors.


Discharging Greywater to Septic Tanks

Sinks, showers and bathtubs may be plumbed to the waste drainage lines in the house and discharged to the septic tank with the rest of the household wastewater. This is the most common and simplest way of managing greywater. To do this, it is important to verify that all plumbing fixtures connected to the drain line are equipped with "P" traps, as shown to the left. A "P" trap is a plumbing fixture that blocks sewer gasses from escaping from the fixture into the indoor environment. Sewer gasses can be identified by their characteristic "rotten egg" odor, and are most obnoxious for residents of the home. For sinks, you can observe the "P" trap easily simply by looking underneath the sink. Observing a "P" for showers and bathtubs is more difficult, as the connection is usually under the floor. If the home is built on a stem wall, observing the plumbing from the crawl space under the floor may be possible. For homes constructed on concrete slabs, there is really no simple way of checking. If tubs or showers give off odors, it may be possible to control this by placing a "P" trap and dedicated vent pipe just before the septic tank. This will generally require the assistance of a plumber.

Discharging greywater to the septic tank adds to the volume of wastewater flowing to the tank, so it is important to verify that septic tanks are properly sized, and have proper outlets. Septic tank outlets should flow to combined stormwater/wastewater drainage canals or soils-based leach trenches. If the drainage from the septic tank is in question or known to be inadequate, consider using a dedicated greywater treatment system.

Dedicated Greywater Treatment Systems

In many cases, plumbing the greywater to the septic tank is impractical, or inadvisable if the septic tank sizing or outlet is in question. In such cases, simple and effective greywater treatment systems can be constructed. These typically consist of a small tank or drum that is equipped with a screen to trap the larger solids. The tank or drum allows the greywater to settle. An outlet pipe from the tank or drum collects the settled greywater a few centimeters below the water surface and sends it either to a soils-based leaching trench, or in the case shown to the left, a small planter box. Here, the plants provide additional treatment of the greywater, and provides an easy way to reuse this valuable water resource. Any remaining wastewater after the planter box is clean enough where it may be piped directly to the nearest stormwater drainage system in full compliance with local laws.

Maintenance for this type of system is easy. Simply clean the screen once a week, and be sure to place the screenings in a covered trash receptacle. Then tend the plants as you would a normal garden. Thin them if the vegetation becomes too dense. Greywater treatment planter boxes are easy and inexpensive to build. It is a nice project for weekend hobbyists, or your local barangay plumbing contractor.


Greywater Treatment and Reuse

Greywater, once it is treated, can be used for various purposes including toilet flushing and landscape irrigation. Greywater may be collected after the treatment drum and planter box in a tank or drum for storage and use by bucketing or as directly plumbed systems. If greywater is intended for reuse in the home, add a small amount of household bleach to the greywater tank. This will help control bacteria that can cause slimy buildups in the tank and also address odors that can be generated from greywater. Use 1/4 cup of household bleach once per week for a 100 liter tank for bacteria and odor control.


Discharging Greywater To City Sewers

Ideally, cities will have centralized sewer systems that can accept wastewater (including greywater) from houses. Unfortunately, in San Fernando City, such sewer systems are years away. Instead, a network of drainage channels that combine wastewater with stormwater exist that transport wastewater to streams, rivers and eventually the sea. Properly treated greywater may be piped directly to these drainage channels without causing any nuisances or adverse public health impacts. Untreated greywater may in some instances be piped directly to drainage channels, as some of these are being connected to sewer treatment plants. Check first with the city engineering department for the specific rules for your drainage area.