How to Plan a 10–50 KLD Faecal Sludge Treatment Plant (FSTP)

An FSTP is sized on the volume of septage that will actually arrive, not on the town's sewage generation. The estimate rests on three numbers: population served by septic tanks and pits, average tank volume, and the desludging cycle. A town of 100,000 people, 90 % on septic tanks of about 3 m³, desludged once every three years, generates roughly 100,000 × 0.9 / 5 persons per household × 3 m³ / (3 × 365 days) ≈ 50 KLD if every tank is actually emptied on schedule.

In reality, unless the urban local body runs scheduled desludging, only 20–40 % of tanks are emptied in a given year and the arrivals are highly seasonal. So a design capacity of 20–30 KLD, with storage to absorb a bad week, is more honest than 50 KLD. Count the trucks: a 3 m³ or 5 m³ vacuum tanker doing two to four trips a day tells you more than any per-capita formula. Design for peak trucks per day, and check that the receiving bay can turn a truck around in ten minutes.

The liquid fraction after dewatering is still five to ten times stronger than sewage. Whatever the town's STP does, the FSTP's liquid stage must be designed for that strength or the effluent will not meet reuse norms.

For a small town with land, planted drying beds are hard to beat: no moving parts, the sludge mineralises over years and the leachate volume is small. Where land costs more than equipment, or arrivals are concentrated in a few hours a day, a screw press with polymer dosing and a buffer tank gives a plant that fits on a quarter of the area.

The filtrate and the supernatant are strong, ammonia-rich and full of pathogens. Options in order of complexity: anaerobic baffled reactor + anaerobic filter + horizontal planted gravel filter (DEWATS) for near-zero power; MBBR or SBR where land is short and effluent limits are tight; co-treatment by dosing into an existing STP at 1–3 % of its inflow where one exists within reach. Every route ends with disinfection — chlorine or UV — and a polishing filter so the water can be reused for landscaping and construction.

Dried sludge from drying beds after 12–24 months of storage, or pressed cake co-composted with municipal organic waste for 6–8 weeks at 55 °C, is safe as a soil conditioner. Solar greenhouse dryers and pyrolysis units produce a pathogen-free product faster where there is a market for it. Plan the end use before you build — a shed full of unsold cake is the most common FSTP failure mode.

• Land: 0.5–1.0 hectare for a 20–30 KLD drying-bed plant; 0.15–0.3 hectare for a mechanised plant of the same size.
• Receiving bay with screen, grit trap and a hardstand that trucks can wash down; odour control by design (covered channels, buffer planting).
• Storage/equalisation of at least two days of peak arrivals.
• Laboratory bench for TS, pH and helminth-egg checks; a weighbridge or flow meter to log every truck.
• Operator housing, power backup and a water supply — many FSTPs are on the edge of town with none of the three.
• Budget line for two years of O&M and scheduled desludging promotion; a plant without trucks arriving is a monument.

If an STP with spare capacity exists within 20–30 km, a septage receiving station is often the fastest and cheapest FSSM solution: a covered discharge bay, coarse screen, grit and grease removal, a 2–3 day holding tank and a dosing pump that feeds septage into the STP inlet at a controlled 1–3 % of plant flow, avoiding the shock loads that otherwise upset the biology. It needs an honest check of the STP's spare capacity for solids and nitrogen — and a sludge line that can take the extra load.

Truck survey and load estimation, technology memo with land and OPEX comparison, detailed design, supply, erection and commissioning, operator training and first-year O&M support — as a civil plant, a modular skid or a containerised unit. See the faecal sludge treatment plant page for the process in detail.