Non-Planted Filters

Design of a Non-Planted Filter

In a non-planted filter, solids are removed by filtration and sedimentation, and organic matter degradation occurs by microorganisms that colonise the filter bed. Chemical adsorption of pollutants onto the media surface also plays a role in removal of some chemical constituents (e.g. phosphorus). The basic structure consists of a watertight box filled with granular material. The most usual depth of vertical filters is 1 to 1.20 m. However, if there is enough natural slope and good ventilation of the influent, vertical filters can also be built up to a depth of 3 metres (SASSE 1998). Hydraulic loading ranges between 40-60 L/m².d, while space needed ranges from 5 to 10 m²/PE depending on the design and the goals of the treatment (CRITES & TCHOBANOGLOUS 1998). 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. Before the pre-treated wastewater or greywater enters the filter body, a pre-treatment is very important to remove oil, grease and solid materials. Septic tanks or anaerobic reactors are a common solution for primary treatment. A properly operated non-planted 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).

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 removed regularly (depends on system) and managed correctly. The filter bed maintenance becomes more complicated when it is covered (UN-HABITAT 2008). As for constructed wetlands, the most important concern is clogging. With time, the gravel will become clogged with accumulated solids and bacterial film. The material may have to be replaced every 8 to 15 or more years. The pre-treatment facility must be maintained and emptied regularly.

Capital costs are moderate compared to intensive treatment options. They are mainly related to land requirements and purchase. Costs for materials are limited if they are locally available (SANIMAS 2005). Furthermore, use of feeder pumps increases operational costs (SANIMAS 2005) as well as maintenance and periodically emptying of the pre-treatment facility (e.g. septic tank). Considering capital and operation and maintenance costs, non-planted filters are still less expensive than intensive treatment options (i.e. activated sludge).

Since the 1950s, sand and gravel filters have enjoyed a resurgence of interest and today they are among the most successful methods for onsite wastewater treatment over the world. Their capability for nutrient and pathogen removal, their low maintenance and power requirements, and their tolerance for periodic surges in loading rates make them practical and economic. Non-planted filters are very common in the United States of America. CRITES & TCHOBANOGLOUS (1998) reported several successful experiences of non-planted filters implementation to treat wastewater in small communities since 1940s. Sand was the mostly used medium while septic tanks or settling tanks and solid separation were the most used pre-treatment. Drip irrigation was the most common method for effluent reuse. Removal efficiency of these systems was about 90-99% for BOD₅, 50-80% for ammonia and 15-50 % for total nitrogen.

Non-planted filter are frequently and successfully implemented in India, especially for greywater treatment. UNICEF and NEERI along with Government and Non-government partners have constructed six greywater treatment plants in Dhar and Jhabua, two districts of Madhya Pradesh in Central Province of India (NEERI 2007).

In these systems the greywater is treated using primary, secondary and tertiary treatment technologies (GODFREY et al. 2009):

• Primary treatment consists of absorption of soap suds using a synthetic sponge, sedimentation baffled/graded settlement tank;
• Secondary treatment involves filtration of water using gravel (10–60 mm size) and non-planted sand filter;
• In tertiary treatment the effluent is treated using aeration and chlorination before being pumped to an overhead tank for reusing.

These plants treat between 1,000 and 4,500 L/d. The treated greywater is reused in flushing toilets and irrigation. The operation and maintenance of these greywater treatment plants are looked after by students and Parent Teachers Association (PTA) (NEERI 2007).

The performance evaluation of these plants carried out by NEERI, showed that the turbidity removal was about 50% (<200 NTU) and that the filtration treatments removed up to 80% of microbial pollution (NEERI 2007).

The financial feasibility of greywater reuse system was undertaken based on a case study of one Girls boarding school in Ganganagar, District Dhar of Madhya Pradesh (GODFREY et al. 2009). The construction costs (material and labour costs) and operation and maintenance costs were considered, as well as the internal benefits generated by the reduction in water consumption. Results showed that the cost of the system might be recovered in two years furthermore (GODFREY et al. 2009).

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