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close this bookGATE - 1/95 - Waste Water: Resource Management and Environmental Hygiene (GTZ GATE, 1995, 56 p.)
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Using of root-zone wastewater treatment plants

Successes achieved with a pilot plant in Berlin by Harald Kraft

Decentralised wastewater treatment systems have been tested in Germany for some years now. They are feasible solutions for the treatment of domestic wastewater. Our author describes a plant in Berlin and how a root-zone wastewater treatment plant can be used in urban conglomerations.

Five systems

Using plants for wastewater treatment has been further developed and tested since the mid 60ies. There are five major systems: (see figure 1).


Wastewater treatment using plants

· A flooded basin fully covered with helophytes (i.e. plants which grow on swampy soils) with a horizontal percolation (or flow).

· Rhizomes (shoots) of helophytes attached to swimming grids and hanging freely in water.

· Root-zones of soil filters covered with helophyte growth, and vertical percolation (or flow).

· Root-zones covered with soil filters with helophytes with a vertical percolation.

· Layer of fine gravel with helophytes on the soil and horizontal percolation.

The systems can be combined in numerous ways. There are hardly any limits using plants to purify wastewater either before discharging it into a watercourse, or purifying it in the body of water itself.

Nowadays plant-based sewage treatment facilities are used to purify groundwater, rainwater, surface water, domestic and industrial wastewater, seepage (from solid waste and sludge dumps) and to dehydrate and process sewage sludge.

Purifying wastewater

The wastewater pollutants are degraded in three stages:

1. by the metabolism of micro-organisms;
2. the metabolism of the plants (helophytes);
3. by solidification in the soil and filter material.

Oxygen and other root discharges flow via the plant roots into the soil saturated with wastewater. The inflow of oxygen allows the existence of both aerobic and anaerobic organisers. The remaining root discharges are assumed to bring about a chemical change in the soil in turn which changes the quantity and quality of soil flora.

It is assumed that pathogenic germs existing in the root-zone can be very effectively eliminated, root discharges with an antibiotic effect (reed) being involved in this.

The capacity to take up organic materials and heavy metals and to transform organic substances are other qualities of the plant. The root growth considerably changes the permeability of the soil water.

Hygiene risks reduced

The operation of a root-zone sewage treatment plant generally requires that the wastewater is well pre-purified, including a sludge treatment stage. This is possible in an Imhoff-tank or a septic tank. The root-zone sewage treatment facility requires a large surface area, estimated at a minimum of 5 m² per inhabitant (in Germany). To stop mosquitos from breeding, care must be taken to ensure that no sewage can come to the surface.

The advantages of the root-zone sewage treatment plant are its simple construction, low energy consuption, lack of any machinery, and the minimum maintenance required, which together keep running costs low. The purification capacity is comparable to that of conventional biological treatment plants. No other treatment system, however, can ensure such extensive elimination of pathogenic germs without applying additional chemical or physical processes.

Pilot project in Berlin

Another great advantage is that the treated sewage can be recycled in agriculture and reduces hygiene risks in the groundwater and surface water.

In 1987 the International Building Exhibition (IBA) in Berlin/Germany, planned and constructed an environmentally compatible housing and urban development project. The project aimed to demonstrate how drinking water consumption, as well as the ecological damage caused by wastewater could be reduced by the conscious management of the resource water.

The newly-built housing complex in Block 6, Berlin-Kreuzberg, which consists of 106 residential units and a 10,000 m² inner courtyard were planned and built to demonstrate the following: reduced water consumption; reduced energy consumption; utilization of rainwater; treatment and reuse of domestic wastewater; groundwater recharge through percolation of treated wastewater and excess rainwater overflow; integration of the root-zone sewage treatment facility into a recreational area; reduction in the amount of solid waste.

Design of the sewage treatment facility:

- an Imhoff (septic) tank for preliminary treatment,
- a root-zone treatment plant for biological treatment,
- a polishing pond for the final treatment.

The domestic sewage of 73 apartments of the pilot project in Berlin is pumped from a collector pit outside the building into the Imhoff tank. The remaining apartments are directly connected to the public sewerage.

Preliminary Treatment

The Imhoff tank was constructed with two pre-fabricated round basins with a filling volume of 34 m³ each and one channel-type aluminium settling tank with a settling area of 5 m³. The basins are covered and the digestion tank is aerated over the roofs of the new buildings.

The root-zone sewage treatment plant consists of 4 modules, operated in parallel, each with a design load of 50 PE (population equivalents). The modules are constructed with waterproof reinforced concrete and divided into 4 sub-basins. Each module inlet is provided with an inlet channel through which treated wastewater is passed through a dentate sill into a feeder channel, which in turn distributes the wastewater over the entire surface of the next filter.

Clumps of Phragmites australis (reed) is planted in filters of the first two sub-basins. The filter of the third sub-basin is planted with Schoenoplectus lacustris (bulrush) in clumps. At the filter of the fourth sub-basin, Phragmites australis (reed) is used as the plant-cover. The difference between inflow and outflow levels yields a medium flow gradient of 2.2 %.

The polishing pond is a waterproof reinforced concrete tank with a volume of 105 m³ and a mean depth of 1.0 m. The clarified effluent is channelled to two pumps. The first pumps into the domestic water tank of the residential block, and the second is used for groundwater recharge.

Satisfied local population

The treatment facility is also a place of interest to many visitors. The bridges, the pond, and the paths within the treatment plant are attractions for the children who come to play here. The hill of the Imhoff tank is used as a lawn or as a playground. The benches next to the rainwater pond are used very frequently. The growth of the trees around the treatment plant is very satisfactory. The children rarely walk inside the zone which is charged with wastewater.

It is remarkable that there have been almost no cases of vandalism and that the amount of litter found in the whole area is negligible.

According to surveys by a research project of the Technical University of Berlin, 97 % of the inhabitants feel comfortable in their apartments (57 % feel comfortable' 40 % feel very comfortable). Thus the wastewater treatment plant in their leisure area does not disturb them.

In March 1988, the treatment plant was fed with the wastewater of 250 inhabitants (2.4 m²/cap.). However, since March 1989, having been overloaded during the preceding winter, the wastewater was reduced to that of approximately 200 inhabitants (3 m²/cap).

The concentration of pollution load is very high. During the first year of operation it was 40 % higher than the concentration of inflows of public treatment plants. During the growing season in spring, the minimum requirements were surpassed. In winter months, however, the minimum requirements could not be met during times of high load by a specific area of 3 m²/cap.

The high percentage of denitrification, the approx. 60 % retention of phosphorus, and the widespread elimination of germs are worthy of note. Despite the high charge by faecal bacteria, the effluent of the polishing pond is almost within the limits set by the guide-lines of the European Community for bathing water.

The emission of faccal odours is negligible throughout the year and is perceptible ouly in the region of the inflow duct. Altogether, the emission of wastewater odours is lower than expected by the enginecrs.

After 1990, the load was reduced such that the effluent is within the limits set by the guide-lines of the EC for bathing water. Since 1992 the waterclosets of approximately35 apartments have been supplied with treated wastewater. In 1993, the distribution of purified wastewater was extended to 72 apartments.

Economising Drinking Water

The concept of the Berlin plant-based water treatment facility, which recycles purified water from bathtubs and showers and uses it to flush toilets has proven to be a success. Erwin Nolde, scientist at the University of Berlin, provided back-up assistance to the pilot project and can put forward the following calculation:

"The maintenance and operation of a grey-water treatment plant to take the wastewater for 60 people requires an input of approximately DM 3,500 per year. Compared with the price of "only" 7 Marks per m³ for drinking and wastewater this constitutes cost savings of about DM 7,500. Assuming an average of 2.5 persons per housing unit, investment costs of about DM 1,700/ housing unit (or 700 DM/ person) should be calculated - hardly more than is needed to connect to cable-TV, which society is well-prepared to do. The ecological benefits of such a facility are enormous. Within one year, we calculated that more than 1,000 cubic metres of drinking water were economised and the load on waterworks and sewage-works was lowered accordingly."

Source: Politische Okologie, Special Issue No. 5, Munich 1994

Rm

Les installations d'assainissement onctionnement dntraliseprntent une option ide pour les rons rurales mais conviennent lement pour les agglomtions urbaines. Ceci est mis endence 'exemple d'une installation d'ration biologique mise en place dans la capitale allemande Berlin. L'eau rest utiliscomme eau de che.

Extracto

Las instalaciones descentralizadas no solo son ideales para eliminar aguas residuales en zonas rurales. Tambipueden utilizarse en grandes zonas urbanas, como lo demuestra una instalacie fitopurificaciue opera en la capital alemana de Berlin. El agua depurada se usa para evacuar excrementos de los inodoros.