Design Report

Pilot Plant

•  Treatment system

For the general scheme of the chosen wastewater treatment option see the drawings “Tav.1-General Layout” and “Tav.2-Sections”.

•  Pumping well (B101)

The treatment system is located in a very flat area. Use of a pump to feed the whole constructed wetland treatment system, including the primary treatment (Imhoff tank) from the existing sewer is necessary. An alarm unit with horn and floater shall be provided (acoustic alarm) to ring an alarm in case of failure of the pump. For the dimensions and details of the Pumping see drawing ”Tav.3-Imhoff Septic Tank and Pumping Station”.

Submerged faecal pump:

•  Inlet well (B102)

The inlet manhole contains a 90° curve for sampling of the inlet raw wastewater. The size of the manhole, realised in concrete, is 120 cm width x 100 cm length x 75 cm height (see drawing “Tav.3-Imhoff Septic Tank and Pumping Station”).

•  Pretreatment Unit - Imhoff tank (B103)

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.

For pre-treatment an Imhoff tank with an effective volume of 20 m³ is planned (see drawing “Tav.3-Imhoff Septic Tank” for more details). The sludge settling in the digestor 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 2/3 of the useable height in the first chamber.

Retention time

Imhoff tank: = 20 m³ / 25 m³/d = 0.8 days

•  Partition (Splitter) well (B104)

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 “Tav.3-Imhoff Septic Tank”).

•  Horizontal flow constructed wetland – (SFS-h) (B105 + B106)

•  Description of the constructed wetland

SFS-h (submerged horizontal flow) systems consist of basins containing inert material with selected granulometry (see drawing “Tav.4-SFS-h 1 system Plan and section (10x20)” and “Tav.5-SFS-h 1 system Technical details (10x20)”). 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) :

•  Functioning

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 .

•  Partition (Splitter) well (B107)

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 Tav.3-Imhoff Septic Tank).

•  Siphons well (B108)

For the alternate feeding of the 2 nd stage Vertical Flow CWs there's the need of siphons (see drawing Tav.7-SFS-v 2 system Technical details (12,5x68) – Feeding well).

The tube-valve causes the rising of pre-treated water in the well up to a certain water level. Reaching that level it tips down and empties a certain quantity into the reed bed. The tube is fixed at the effluent pipe, part for lifting like a float switch with the rising water level. Reaching a certain angle it remains in position and the water enters into the tube. When the weight of the entered water exceeds buoyancy a mechanism in the valve opens and the moving part of the tube-valve tips down, thus releasing the water in the well down to ground level.

•  Vertical flow constructed wetland – (SFS-v) (B109 + B110)

In the vertical flow systems (VF) the wastewater is applied through a distribution system on the whole surface area and passes the filter in a more or less vertical path (see drawings “Tav.6-SFS-v 2 system Plan and section (12.5x68)” and “Tav.7-SFS-v 2 system Technical details (12,5x68)”). The pre-treated wastewater is dosed on the bed in a large batch (intermittent feeding), thus flooding the surface. During the time between the feedings the pores within the filter media can fill up with air which is trapped by the next dose of liquid. Thus oxygen requiring nitrifying bacteria are favoured and full nitrification can be achieved, but only a small part of the formed nitrate is denitrified under aerobic conditions. The treated water is collected in a bottom drainage system to be discharged to the following stage of the treatment. The water level can be maintained with a height of about 5-10 cm from the bottom of the bed, or otherwise the beds can be totally empty after each feeding pulse.

This kind of CW is particularly efficient in nitrification, carbon and suspended solids removal. Due to its prevalently aerobic conditions denitrification is poor.

SFS-v (submerged vertical flow) systems consist of basins containing different layers of inert material with selected granulometry. The bottom and the walls of the basins has to be correctly waterproofed 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 systems consist of two general collector pipes, on the bottom and the top of the bed, connected to several smaller pipes for spreading the water over the whole surface and collecting it from the bottom. The drainage system is connected to the outlet well allowing to empty the bed after each pulse of wastewater. Emerging plants, generally reed ( Phragmites australis ) are grown in the basins.

Table 3 : Technical Specification of the Vertical Flow Constructed Wetland – (SFS-v) :

•  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 0.5%;

•  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;

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

•  Put the feeding system;

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

•  Horizontal flow constructed wetland – (SFS-h) (B111 + B112)

Same description and procedures of B105 + B106 (see drawings “Tav.8-SFS-h 3 system Plan and section (12,5x60)” and “Tav.9-SFS-h 3 system Technical details (12,5x60)”).

Table 4 : Technical Specification of the Horizontal Flow Constructed Wetland – (SFS-h) :

•  Sludge treatment constructed wetland (B113+B114)

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 Tunisian 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 recirculated by a drainage system to the first stage horizontal flow constructed wetland (B105).

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 “Tav.10-Sludge drying reed bed”).

Table 5 : Technical Specifications of the Sludge Treatment Constructed Wetland :

Cost

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

Table 6 : Construction and maintenance cost of the proposed system (TD and EUR per year)

The construction cost are valid under the assumption that the works are carried out by ONAS under the supervision of the CERTE researchers.