Sustainable Concepts Towards a
Zero Outflow Municipality

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•  Treatment system

•  Pretreatment Unit - 3 - Chamber pretreatment tank (A101)

To reduce the amount of solids in the inflow and to minimize the risk of clogging of the filter bed a pre-treatment is required.

According to Austrian Standards, ÖNORM B 2505, “The minimum volume for the pre-treatment unit should be no less than 0.25 m³/PE. The surface area has to be at least 0.06m²/PE. The minimum water level has to be at least 1,30m.

For pre-treatment a 3-chamber septic tank with an effective volume of 56 m³ is planned. In order to improve the treatment efficiency the pre-treatment unit will be divided into 3 chambers (see drawing “02_SEKEM Pretreatment” for more details). To increase the mechanical treatment effect dip pipes have to be installed between the pre-treatment chambers. The sludge settling in the first chamber has to be removed from time to time. It is transferred to the sludge treatment constructed wetland. Removal becomes necessary if the sludge volume reaches a height of 1/3 of the useable height in the first chamber.

Retention time

Stage I: = 56 m³ / 20 m³/d = 2,8 days

Stage II (Extension): = 56 m³ / 30 m³/d = 1,9 days

•  Pumping well (A102)

The treatment systems is located in a very flat area. Use of a pump to feed the constructed wetland from the pre-treatment unit is necessary. After this point the wastewater will flow by gravity trough the treatment system to the storage pond. An alarm unit with horn and floater shall be provided (acoustic alarm) to ring an alarm in case of failure of the pump.

Submerged faecal pump:

•  Splitter well (A103)

For optimal and accurate distribution of the water on the CW a splitter well has to be provided. An installed divider weir splits the incoming water flow into two equal flows in stage I and into three equal partial flows in stage II (expansion stage). The weir is made of stainless steel. (see drawing 04_SEKEM splitter well).

•  Horizontal flow constructed wetland – (SFS-h) (A104)

•  Description of the constructed wetland

SFS-h (submerged horizontal flow) systems consist of basins containing inert material with selected granulometry. The bottom and the walls of the basins has to be correctly waterproofed using a layer of clay, if available on site and under adequate hydrogeological conditions, or, as is more frequent, using synthetic membranes (HDPE or LDPE, 1,5 to 2 mm thick, or a sandwich of a thinner membrane with two layers of geotextile) or exceptionally concrete. The inlet and the outlet consist of two strips of coarse material along two opposite edges of the basin. At the inlet side a feeder pipe with large holes (e.g. Ts) at regular intervals runs along the edge near the surface. In the outlet gravel pack a drainpipe is put at the bottom to collect the treated water. The drainpipe goes into a riser pipe in a shaft. The height of the opening of this riser pipe determines the outlet water level in the basin. The path of any water percolating through the basin should have a length between 3 to 10 m. Accordingly water is either fed on the short (for small basins) or on the long side of the basin. Emerging plants, generally reed ( phragmites australis ) are grown in the basins.

Table 2 : Technical Specification of the Horizontal Flow Constructed Wetland – (SFS-h) :

•  Function:

The inert material in the basins has to provide a filtering effect and growth support for microorganisms but also assures an adequate hydraulic conductivity (filling media mostly used are sand and gravel); these inert materials represent the support for the growth of the roots of the emerging plants.; The water remains always under the surface of the absorbing basin and flows horizontally thanks to a slight difference between the inlet and the outlet levels. The filling material of the basin is saturated with water but for the uppermost few centimetres.

During the passage of wastewater through the ryzosphere of the macrophytes, organic matter is decomposed by microbial activity, nitrogen is denitrified, if in presence of sufficient organic content, phosphorus and heavy metals are partly fixed by adsorption on the filling medium; the contribution of the vegetation to the treatment process can be represented both by the development of an efficient microbial aerobic population in the ryzosphere and by the action of pumping atmospheric oxygen from the emerging part of the plants to the roots and so to the underlying soil portion, with a consequently better oxidation of the wastewater and creation of an alternation of aerobic, anoxic and anaerobic zones. This succession of zones leads to the development of different specialized families of micro organisms and a good reduction of pathogens, highly sensitive to rapid changes in dissolved oxygen content.

•  Chronology of construction and technical details

•  Along the edges of the bed, dig a small trench to tuck in the geotextiles and the liner ;

•  Cover the bottom and the banks with a nonwoven geotextile (minimal density 250 g/mq);

•  Put a sand layer on the bottom of the bed to achieve a slope of 1%;

•  Put the PE liner and pass the pipes through the liner as specified in the drawings;

•  Cover the bottom and the banks with a second layer of geotextile (minimal density 250 g/mq);

•  Put the edges of the sheets into the boundary trench and backfill with the excavation material;

•  Put the drainage system;

•  Put a rock layer on the drainage system;

•  Put a rock layer (50 cm high) in the inlet zone;

•  Put the feeding system on the rock layer and cover with rock until the designed height, as specified in the technical drawings;

•  Fill the bed with gravel: it is strongly recommended that the gravel be well washed and round. The final filling surface must be horizontal, i.e. have no slope towards the outlet.

•  Plant the reeds in the gravel, with a density of 4 plants/m 2 .

•  Sludge treatment constructed wetland (A106) and leachate treatment (A107).

Uncontaminated sewage sludge from domestic waste water contains a lot of nutrients and organic material, which, after a hygienisation, can be used as high quality fertilizer in agriculture.

The drying and composting of sewage sludge in a CW means dewatering, volume reduction and mineralisation, stabilisation and hygienisation of the material without chemicals and additional energy input. Due to dewatering, decomposition and compaction of the material the volume of the applied sludge is reduced by 85 % during the operation cycle. The total nitrogen content is reduced by about 50 %. Important minerals for agriculture like potassium, calcium and magnesium show only a slight decrease in relation to the original mineral content (sodium unfortunately does, too).

The final product “mineralised sludge” is a crumbly, light brown-coloured material with a typical earthy smell. Organic components and pathogenic micro-organisms are reduced and the final product is suitable as fertiliser and soil conditioner in agricultural.

The volume is chosen in order to allow a 10 year filling period with a 0,5 m sludge layer at the end of this period, taking into account a dry matter content of 30 to 40% (under Central European climatic conditions) and a 85% volume reduction. Under Egyptian climatic condition a higher dry matter content can be expected.

The CW are batch fed to maintain aerobic conditions within the filter beds. The reed improve microbial growth, assist in the prevention of clogging and create a large drying and aeration network. Aerobic conditions on and in the filter prevent the emission of smell. Sludge accumulates on the surface. The leachate infiltrates into the filter and is led by a drainage system to an attached horizontal flow constructed wetland (A107). The treated leachate will be collected in the storage pond for irrigation.

•  Sludge treatment constructed wetland (A106)

The decommissioned vertical flow constructed red bed (A106) of the old treatment plant shall be adapted to the sludge treatment constructed wetland. The dimensions and volume of the structure perfectly qualifies it for this purpose. The structure has to be renovated and the tightness checked and restored where necessary. The polluted filter material has to be removed and a new drainage and filter layer has to be implemented.

An outlet water control device for the sludge treatment constructed wetland has to be installed. The operating water level is fixed at 0,24 m. The outlet control device allows to completely empty the filter bed and gives also the possibility to flood the bed for weed control during the start up phase. The water level control device is made from PVC pipes and fittings (for details see drawing 07_SEKEM Sludge drying reed bed).

Table 3 : Technical Specifications of the Sludge Treatment Constructed Wetland :

•  Leachate treatment (A107):

The leachate of the sludge treatment CW has to be treated before discharging into the storage pond.

The decommissioned horizontal flow CW of the old treatment plant shall be adapted for the leachate water treatment. For this propose the old polluted filter material has to be removed and washed. The cleaned filter material (gravel) can be used again in the CW. A replanting of the filter with reed after the refill of the cleaned gravel is necessary.

Technical specification of the leachate treatment :

•  Storage Pond (A108)

The existing storage pond (A108) shall be further used for the collection and buffering of treated water. The water will be used for irrigation. For a thorough cleaning it will be necessary to completely empty the pond. The tightness has to be checked. A partial renovation and sealing of the pond may be required.

•  Cost

According to our forecast, the investment and the maintenance costs for the proposed system are reported on table 4.

Table 4 : Construction and maintenance cost of the proposed system (EGP and EUR per year)

The construction cost are valid under the assumption that the works are carried out by SEKEM under the supervision of the farm engineer.