Properties situated off the mains drainage require a form of wastewater treatment or an upgrade of the original installed solution. The variety of choice available can be daunting.
Previously, the wastewater from a remote property would have flowed to either a cesspool or a septic tank. A cesspool is an underground holding tank with no outlet, which stores the wastewater until it is time to have it emptied. A septic tank provides partial treatment, settlement and separation of the solid waste and has an outlet pipe, normally discharging to a sub-surface soakaway or ditch.
Present-day water usage means that cesspools are an expensive solution due to the frequency of emptying, whilst septic tanks are becoming increasingly unpopular due to the poor quality of the water discharged and the subsequent negative impact on the environment. This is why modern, small scale, wastewater treatment systems have become the preferred solution.
Most of these systems are based on biological treatment, consisting of digestion of the organic matter by highly efficient, oxygen reliant (aerobic) bacteria. The aerobic process provides treatment to a much higher standard than a septic tank, which means that the treated water can be discharged to a soakaway in less porous soil or, with permission from the environmental regulator, directly to a stream or river.
When planning a wastewater treatment solution, in most cases it is necessary to apply to the environmental regulator for consent to discharge. The Environment Agency (EA) in England and Wales, the Scottish Environmental Protection Agency (SEPA) in Scotland and the Department of the Environment (DOE) in Ireland will set the required standard of water quality, typically based on the amounts of Biochemical Oxygen Demand (BOD), Suspended Solids (SS) and Ammoniacal Nitrogen (NH4) permitted in the treated water (final effluent). This will provide guidance in the choice of treatment solution; in most cases, the standard will dictate the need for a package treatment plant. The plant chosen should also be CE marked and conform to the building regulations (part H2 in the UK) and the new European standard EN12566-3, which describes the minimum requirements for small package sewage treatment plants.
With the regulatory issues satisfied further matters of cost, installation and maintenance should be considered. The cheapest system to buy will not necessarily be the most cost effective to install when the costs incurred for heavy machinery and the amounts of installation material required, as well as running costs determined by the emptying (de-sludging) intervals and the complexity of maintenance are considered.
When assessing the installation requirements, the regulations stipulate that the plant has to be installed at least seven metres away from any property. The shape of the treatment plant determines the size of hole required for the installation. For example, a conical shape will require less space in the ground and therefore the cost of digging and disposal of the dug out material will be lower than for a plant that is square or cylindrical. The backfill material will need to be chosen to suit the manufacturer’s recommendations and prevailing ground conditions. Most tanks have to be fully encased in concrete, which raises the installation cost, but some are robustly manufactured and can be installed using pea gravel or shingle. Problems can arise in areas with a high ground water level, which may mean that the tank will need to be anchored into the ground or surrounded in concrete.
A large aspect of the installation process is the creation of a point of discharge into which the treated effluent can be dispersed. Discharge options include a sub-surface soakaway, raised bed soakaway, drainage ditch, seasonal soakaway, constructed wetland or a watercourse, all of which are designed to suit the conditions in the land and the ground water levels. A soakaway is the environmentally preferred point of discharge because the treated effluent receives physical filtration and further biological treatment before it merges with the ground water, but the size and type of a soakaway must be calculated from the results of a percolation test and soil assessment.
The different manufacturers of package sewage treatment plants offer varying warranties on the plants and the additional machinery. Warranties on the products vary widely between manufacturers, therefore the research on the best warranty also is an important cost consideration. It is also crucial to consider the inner structure of the plant as plants with internal mechanical moving parts potentially create more problems than plants which have all mechanical moving parts externally. These are less exposed to failure and therefore ensure peace of mind.
Following the simple process of choice it will be easy to determine which wastewater treatment plant is the best for each job.
