Key note: Drying beds Key note speaker Mbaye Mbégueré

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1 Key note: Drying beds Key note speaker Mbaye Mbégueré
Online Course Faecal Sludge Management Developed by UNESCO-IHE and SANDEC January 2016 Key note: Drying beds Key note speaker Mbaye Mbégueré Project Coordinator at Office National de l'Assainissement du Sénégal

2 FSM treatment technologies
Drying beds

3 Unplanted sludge drying beds (SDBs)
Shallow filters Filled with sand and gravel Contains an underdrain for the leachate Free water evaporates from the surface

4 Treatment principles The drying process is based on two principles:
Percolation of the leachate through sand and gravel. This process is significant with sludge that contains large volumes of free water, and is relatively fast, ranging from hours to days. Evaporation, removes the bound water fraction and this process typically takes place over a period of days to weeks. 50 to 80% by volume due to drainage, and 20 to 50% due to evaporation in drying beds with FS

5 Sludge drying bed facility
contains a number of beds Sludge is loaded into the beds periodically (for example once every 2 weeks) Water is let to evaporate and drain (drain: 50-80%) Upon required dryness -> removed mechanically or manually

6 Unplanted sludge drying beds

7 Unplanted sludge drying bed design
Climate factors : Humidity: high humidity reduces the contribution of evaporation; Temperature: higher temperatures, also in combination with relatively low humidity and high wind, will enhance the evaporation; Rainfall: in locations where rainfall is frequent and occurs for long periods of time intense, a drying bed may not be feasible. Type of fecal sludge: The origin of the sludge is important when using drying beds. Septic tank sludge has less bound water and is hence more readily dewatered than fresh FS. Sludge loading rate kg TS /m2/year -> cm sludge accumulation with 15 days retention time Thickness of the sludge layer

8 Important !!! Local knowledge either from experience or from preliminary drying tests under local conditions. The first stage in conducting drying tests will be to determine the number of days required to obtain a desired TS and water content. If for example the results from these drying tests indicate a two week drying period, including one day for loading and two days for removal.

9 Gravel and sand Both need thorough washing to avoid clogging!
Two diameter sizes of gravel: Small at the top (5-15mm): to prevent sand from washing away Larger at the bottom (20-40mm): to avoiding clogging

10 Gravel and sand Diameter of sand also important: larger grains = larger pores = more accumulation of organic matter -> clogging Better than mm Other prerequisites: sand particles do not crumble sand easily available locally Replacing sand regularly: loss few cm every 5-10 drying sequences Camberene FSTP 5 cm lost after 25 sequences

11 Sludge removal In order for the sludge to be removed properly, it needs to be dry enough that it can be shoveled. Sludge with a TS content of at least 25% fit for removal. The drying time of a specific sludge type depends on a number of factors, one of which is the sludge dewatering resistance. In order to remove the sludge, a ramp must be provided to allow wheel barrows or other equipment to access the bed.

12 Rewetting of the sludge

13 Quality of dried sludge and leachate
The main purpose of a drying bed is to achieve dewatering; i.e. a physical separation between liquid and solids. Drying beds are therefore not designed with stabilization or pathogen removal in mind, although some biodegradation may occur. Therefore, any pollutants present in the FS are not removed and either remain in the sludge or are present in the leachate.

14 Example Camberene Leachate still highly concentrated
2,500 mg TS/L 1,900 mg SS/L 3,600 mg COD/L Dried sludge removed manually: One worker - 2 days for removing 7 cm of dried sludge from a 130 m2 bed. The dried sludge density is about 300 kg/m3 FSTP produces about 600 m3/year of dried sludge Sludge stored behind drying beds, later collected by public works companies for soil enrichment.

15 Sludge excavation - manually

16 Planted sludge drying beds (PDBs)
FSM treatment technologies Planted sludge drying beds (PDBs)

17 Planted sludge drying beds

18 Schematic

19 Schematic

20 Planted sludge drying beds
Same setup, but now with plants on top (and soil on top of the gravel) Plants need to withstand varying watering regimes and nutrient loads Temperate climates: evapotranspiration up to 2.5cm water /day can be achieved in reed stands on very hot days PDBs: > 95% volume reduction over a year (6 months loading, 6 months total sludge loading depth of up to 493 cm; 69% dry matter

21 Plant requirements Fast growing under diverse conditions
High transpiration capacity Tolerance to different water levels and drought conditions extremes of pH and salinity Deep growing rhizome and root system Ability to build new roots on the nodes when they become encased in sludge Readily available, indigenous and non-invasive

22 Types of plants Plant species Common name Water type Habitat
Water regime Phragmitis sp. Reeds Fresh – brackish Marshes; swamps Seasonal to permanent inundation; up to 60cm Typha sp. Cattail Fresh – marshes Ponds Seasonal to permanent inundation; up to 30cm Cyperus papyrus Papyrus Ponds, lakes Echinochloa sp. Antelope grass Seasonal to permanent inundation; up to 40cm

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24 Role of macrophytes stabilizing the beds to prevent media erosion and clogging, and improving the drainage; increasing moisture loss (through evapotranspiration, in contrast to only evaporation in unplanted drying beds); providing a surface area for microbial growth within the sludge layer; transferring oxygen to the sludge layer (i.e. within the rhizosphere); and absorbing heavy metals and nutrients.

25 Operational parameters

26 Operation and maintenance
Commissioning/start-up During the start-up phase, the beds should be irrigated with untreated wastewater or diluted FS. Planting macrophytes during the rainy or wet season is also recommended to help the macrophytes endure the commissioning phase. Loading rates and sludge accumulation In Europe, loading rates with wastewater sludge have generally been low (not more than 80 kg/m2/year) In tropical countries results have revealed that PDBs can be loaded with almost three times this amount Plant haversting and regrowth Harvesting is done by cutting plants at the surface, not by pulling out the whole plant Leachate It should be collected and treated with a subsequent treatment technology prior to discharge to the environment

27 PDBs: crops also have value
E. pyramidalis marketed in peri) urban centres Price varies throughout the year ( USD/kg) Generating daily income between 500–1,000 USD (dry) and 1,600–2,400 USD (rainy season)

28 Important quantity of biosolids generated
> 2000 t DS/ha/an

29 Physical-chemical characteristics of biosolids
Parameters n mean ± StDev Poultry manure* pH’water’ 18 6.01 ± 0.26 8.77 Total N (% DM) 24 2.0 ± 0.2 2.4 Total C (% DM) 22.6 ± 3.2 22.3 Total P2O5 (% DM) 12 2.3 ± 0.6 2.5