Non-planted Filters

Unplanted vertical-flow sand or gravel filters are frequently and successfully applied for pre-settled domestic greywater throughout the world, even in regions with cold winters such as Canada, Germany, Switzerland, Norway or Sweden (MOREL & DIENER 2006). They can also be applied for the tertiary treatment of the effluents from primary or secondary blackwater treatments (e.g. biogas settlers; anaerobic baffled reactors; waste stabilisation ponds; or horizontal flow, vertical flow or free surface constructed wetlands)

The design of a non-planted filter system is very similar to the design of constructed wetlands (see also horizontal flow, vertical flow or free surface). In constructed wetlands, the plants may have a positive effect on breaking up the biological clogging layer on the filter surface, but apart form that there is not much difference in the system performance between planted and unplanted systems. In fact, the sand filter system for greywater purification has been in use for more than 100 years, thus it is a well-known application (HEEB et al. 2007). Non-planted filters are also referred to as subsurface biofilters, percolation beds, infiltration beds or intermittent sand filters (MOREL & DIENER 2006).

Greywater undergoes physical, chemical and biological treatment in a non-planted sand filter. Suspended solids are removed by mechanical filtering and sedimentation, and degradation occurs by microorganisms that colonise the filter bed. Since organic matter and nutrients in the wastewater allow microorganisms to grow and reproduce, the organic matter is mineralised and nutrients are partly removed. Chemical adsorption of pollutants onto the media surface also plays a role in removal of some chemical constituents (e.g. phosphorus, surfactants). Although different materials, such as pea gravel, peat or crushed glass, can be applied as filter media in non-planted filters, sand is the most widespread material used. A combination of different filter material of different granulometry (e.g. gravel, sand and mulch) has also been observed. The combination of the filter material depends on the wastewater quality. For a better phosphorous removal, sand with a higher Fe content is requested (MOREL & DIENER 2006).

Pre-treatment

Before the pre-treated wastewater or greywater enters the filter body, a pre-treatment is very important to remove oil, grease and solid materials (e.g. kitchen waste). Otherwise the risk of clogging the filter bed is high. Septic tanks or Anaerobic Baffled Reactor are a common solution for primary treatment. Also a simple grease trap (see picture below) is an appropriate possibility for single households.

Filter body

The basic structure consists of a watertight box filled with filter material. Greywater is applied to the top of the filter body, percolates through an unsaturated zone of porous material and is then collected in a drainage system. The water is applied intermittently and evenly on the filter surface by a pressure distribution device such as an electric pump or mechanical siphon. By charging the entire surface of the filter, oxygen is supplied to the filter media (MOREL & DIENER 2006).

The most usual depth of vertical filters is 1 m to 1.20 m. However, if there is enough natural slope and good ventilation of the influent (e.g. by sprinkling the wastewater on the filter surface to enhance the contact with air), vertical filters can also be built up to a depth of 3 metres. In the absence of plantation, the surface must be scratched at the beginning of the resting period, in order to allow enough oxygen to enter (SASSE 1998).

Loading of a vertical filter

Feeding in doses is necessary for equal water distribution. The resting times are needed to enable oxygen to enter the filter after wastewater has percolated. Doses must be large enough to completely flood the filter for a short while in order to distribute the water evenly over the surface and small enough to allow enough time for oxygen to enter before the next flooding. Correspondingly, the filter material must be fine enough to cause flooding and porous enough to allow quick percolation (SASSE 1998). Uniform distribution of greywater onto the filter surface is essential and can be achieved either through a dense network of perforated pipes or a sprinkler system (MOREL & DIENER 2006).

Performance

A properly operated non-planted sand filter can produce high-quality effluent with less than 10 mg/l BOD (90–95% removal) and less than 10 mg/l TSS (90–95%). Nitrogen removal is rather limited (30–40%) (MOREL & DIENER 2006). Bacteria, virus and phosphorus removal is enhanced when using sand rich in iron oxides (HEEB et al. 2007). However, the current operational vertical flow filter concepts re-circulate the nitrified effluent from the filter bed back through the anaerobic primary treatment of the system (e.g. septic tank or anaerobic baffled reactor) where denitrification occurs. The recirculating vertical flow filter system is very efficient at removing nitrogen (up to 95%), however, the complexity of the system makes it inappropriate for application on household level in low and middle-income countries (MOREL & DIENER 2006). The treated effluent of a non-planted filter can be used as irrigation water, fertigation water, or for surface or sub-surface groundwater recharge.

Capital costs are moderate. Spatial requirements reduce cost for land purchase. Costs for materials are less if they are locally available (SANIMAS 2005). The pre-treatment facility, which is very important to prevent clogging of the filter bed, must be maintained and emptied regularly. Furthermore, use of feeder pumps increase operational costs (SANIMAS 2005) as well as maintenance and periodically emptying of the pre-treatment facility (e.g. septic tank).

Regular maintenance works at the treatment facility comprises removal of unwanted vegetation from filter bed and cleaning of the inlet/outlet systems. The sludge from the pre-treatment system is desludged regularly (depends on system) and disposed correctly (e.g. drying beds). The filter bed maintenance becomes more complicated when it is covered (UN-HABITAT 2008).

A non-planted filter in combination with a pre-treatment facility is a big improvement for health issues of the community and its surroundings. The effluent of the filter is widely harmless.

The health risk which arises from settled wastewater in the pre-treatment facility should be considered during inspections and emptying. Mechanical emptying in a vacuum truck or a manual technology like a sludge gulper can decrease the health risks (see also human-powered emptying and transport and motorised emptying and transport) (TILLEY et al. 2008).