Sludge Drying Reed Beds for faecal sludge treatment

Sludge Drying Reed Beds, also called Planted Drying Beds (PDBs), consist of the following main components which are also schematically illustrated in Fig.1 below:

• Plastic liner
• Filter layer: sand (5 - 10 cm thick), transition layer with fine and medium sized gravel (15 - 25 cm thick) and coarser gravel (25 - 40 cm thick)
• Vegetation
• Drainage and passive aeration system
• Distribution system, pressure pipes diameter >90 millimetres (mm), with an outlet every 30 - 50 square metres (m2)

Fig. 1: Processes in SDRB. Source: NIELSEN (2003)

SDRBs are divided into several basins (minimum 4 for small applications, generally at least 6), the bottom of the beds is sealed with a waterproof membrane and vegetation is planted in the filter layer. The beds typically have a bottom slope of 1 - 3%, and 1.0 - 1.5-metre-high soil embankments for sludge accumulation, with a slope of 1:1 (or vertical concrete walls).

Treatment process

Faecal sludge is loaded on the surface bed with a Sludge Loading Rate (SLR) of 50 - 200 kilogram per square metres per year (kg/m²/y) of Total Solids (TS), where 7.5 - 20 cm of sludge is loaded 1 - 3 times a week (ENGLUND and STRANDE 2019). Despite some authors reporting high SLR (up to 200 kg/m² or more), the operation of the system at these challenging conditions is still to be demonstrated and might lead to insufficient treatment performance in the short or medium term. SLR of 50 - 120 kg/m²/y of TS are more realistic, with the higher loading rates potentials in tropical and hot climates.

The reduction of the liquid part of the sludge takes place due to percolation through the filter layer, evaporation, or evapotranspiration, while the solid part undergoes a process of mineralization due to the interaction between oxygen, plants, and bacteria. The final treatment products are dried sludge, usually suitable to be used as soil conditioner in agriculture, and a liquid effluent that needs further treatment.

SDRBs can work for five to ten years before this emptying procedure is necessary: the loading of the bed to be emptied is interrupted for about 4 - 12 months to complete the stabilization phase, after which the organic top layer is excavated, and the bed is re-established for the next phase.

The application of SDRBs is possible for faecal sludge with Suspended Solids (SS) content lower than 4% and it is particularly favourable in tropical and hot climates, where the effectiveness of the process is higher. Rainfall must be considered during hydraulic calculations, and since the beds have a limited hydraulic conductivity, an emergency overflow is suggested; however, monsoons are not an issue, as they are limited to a few months and are followed by a dry and warm period. Nevertheless, runoff infiltration in the beds must be avoided. Vegetation growth is favoured in tropical and sub-tropical regions, and SDRBs are successfully used in several dry and warm regions and desert areas (BARRETO et al. 2013).

The main constraint to the implementation of SDRBs is the area they require, and the availability and cost of the land needed. SDRBs are usually used to treat sludge from small and medium communities but are also a suitable option for large scale applications if enough land is available.

For small installations, the proximity to buildings or public spaces may require solutions to mitigate the possible odour propagation (trees, hedges…), while bigger treatment plants are usually placed in peri-urban areas or far from inhabited agglomerates.

The low energy consumption, easy operation and maintenance and low related costs make SDRBs an affordable solution for developing countries.

The operation of the SRDB consist of three phases:

• start-up and commissioning (1 - 2 years), where the SLR is progressively increased, to ensure the vegetation gets acclimatised.
• operation (4 - 8 years), where the sludge is loaded according to the design values.
• decommissioning (1 - 3 years), where loading of one of the two reed beds is stopped for few months and the sludge is extracted by an excavator while the respective other bed is kept in operation.

The operation and maintenance of the SDRBs can be carried out by unskilled workers which are adequately trained based on the Operation and Maintenance Manual. For larger installations, a higher level of experience is required to obtain better results.

The vegetation needs to be regularly harvested. In comparison with unplanted drying beds, the quality of the leachate is much better but still requires to be collected and adequately treated.

BORDA (2018) reports the application of this technology in Leh, a town in the Indian state of Jammu and Kashmir, which is situated at an altitude of 3,524 metres (11,562 ft) and has a population of around 31,000. With a high ground water table being prone to contamination the town’s Municipality Committee decided to implement a sludge treatment plant to treat and reuse the sludge.

A Sludge Drying Reed Bed (10 units, capacity: 12 m³/day/bed) followed by horizontal planted gravel filter and a polishing pond were established with construction cos of Indian Rupees (Rs.) 52 lakhs (about 70.000 US$). The bed was loaded with an SLR of 100 kgTS/m²/y and had a BOD (Biological Oxygen Demand) concentration in the effluent of less than 30 milligram per litre (mg/l).

In Kathmandu, Nepal, a pilot scale experiment was implemented to analyse the performance of SDRBs (PANDEY and JENSSEN 2015). Two SDRB beds with a bed surface of 1.5 x 0.7 m and a depth of 1.0 m and a freeboard of 50 cm above the surface layer were established. Bed 1 was loaded with an SLR of 250 kgTS/m²/y, and Bed 2 with SLR of 100 kgTS/m²/y showing the following results:

• Bed 1 TS content = 21%; reduction rates: volume = 92%; VS = 47%; TKN = 12%; TP = -20%
• Bed 2 TS content = 35%, reduction rates: volume = 96%; VS = 58%; TKN = 30%; TP = 0%

Based on the results from this experiment, an initial SLR of 100 kgTS/m²/y is suggested for tropical climates, and after one year of operation the SLR can be gradually increased.

In Nègrepelisse, the biggest full scale (2,600 m² of SDRB) septage treatment unit in France has been built and been in operation since 2013, with the following key data:

• stone trap + automatic screening + emptying tank + aerated buffer tank + 8 SDRBs + 2 Vertical Flow + treated leachate storage basin
• 8 SDRBs, 325 m² each
• SLR: 50 kgSS/m²/yy
• treated sludge reused for agriculture spreading.
• removal rates (average): 99.5% TSS, 98.3% COD, 94.9% TKN, 94.8% TP
• about 10 cm/year of deposit accumulation; average drying matter around 24% (KIM et al. 2018)

In Zanzibar, Tanzania, a SDRB is included in the Kibele Landfill facility, and was started up in October 2020:

• OLR max 120 kgTS/m²/y
• 12 PDBs of 286 m² each, for a total surface of 3,482 m²
• Filtration layer: 55 cm
• A volume reduction of about 93% is expected after 5 - 7 years (HYDEA 2020)

In Dakar, Senegal, full-scale SDRBs have been in operation since 2008 at the Cambérène treatment facility:

• Bed surface area: 130 m²
• SLR: 80 kg/m²/yr
• Planted with Echinochloa pyramidalis (Antelope grass)

Average sludge dry matter content: 45% (DODANE et al. 2011).