Lime Stabilization – Unplanted drying beds – March 2019

Lime Stabilization – Unplanted drying beds – March 2019
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Location of the case study

Bangladesh - Cox's Bazaar - Kutupalong-Balukhali extension area - camp 18

 

Main treatment objectives

Pathogen reduction

Capex per design input flow

1300 USD/m3/day

Opex per real input flow

17USD/m3

Required space

0.06m2/person

Description of the treatment process

The faecal sludge treatment site is designed with the primary objective of pathogen elimination. It consists of lime stabilisation for pathogen elimination, unplanted drying beds for solid liquid separation and dewatering and a sludge drying area.

Description of the emergency context

The FSM site is situated in a densely populated refugee camp (Cox’s Bazar, Kutupalong-Balukhali extension area, camp 18) with hilly terrain and poor path conditions.

Indicators

Narrative

Lessons learned

Reduce liquid content prior to discharge at treatment to avoid transport of liquids and frequent emptying, ensure latrine design allows for manual desludging, increase desludging capacity and design low-tech treatment options with quick start-up time and ensure pathogen kill that also is modular for future purposes (up-scale, climate modification etc.).

Desludging and transport is coordinated with available treatment capacity, but it was a bottleneck during dry season when drying capacity is fast.

Faecal sludge treatment is never better than the latrine design component, desludging capacity or transport situation. With change in climate, latrines could be flooded, infiltration will reverse, transport and access might be reduced or impossible. During rainy season the treated faecal sludge volume could stay fixed, but people served decrease since liquid content possibly increase. Once again, faecal sludge is a complex process well integrated and dependent on the full faecal sludge chain.

The treatment facility will never perform better than the weakest link in the faecal sludge chain.

– Large scale mixing of dissolved lime and faecal sludge can be a challenge since sufficient mixing is crucial for an efficient pH increase. When receiving larger volume of faecal sludge the logistics of handling 50 barrels might be more tricky than finding a way for large scale mixing of lime.

– Lime and alum works as coagulant and improve settling properties. When applied at large scale in a 1000 litre overnight (attempt to a settling tank), the challenge were to discharge the solids and liquids separate without short circuiting and mix-up solids and liquids again. Idea is to discharge liquids first prior to solids, just need to address increase in viscosity of solids without the hydraulic head from liquids on top. Settling requires screening to prevent blockages, and liquids should be decanted from the top of the tank first to prevent short circuiting. This can be achieved using a flexible hose that is connected to an outlet at the bottom of the tank. The hose can be lowered into the liquid layer where it can drain down the hose and out to a soakaway.

– Infiltration beds with gravel as only filter media clogs rapidly of the solids.

– Drying time will fluctuate with changes in climate.

– Local sandbags work on small scale for solid liquid separation. To try on large scale.

– Local rice bags were to compact to allow rapid infiltration on large scale, drying bed ponded.

– Measurements days after discharge to drying beds show pH 12.

– Regardless of what technology is employed, a treatment site is ultimately a sludge factory. Therefore safe and sustainable disposal routes need to be further development and investigated (i.e. appropriate fertilizer use in surrounding agricultural areas should be researched and markets developed).

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9 months ago

Thank you for sharing this helpful post with us.