For a small landfill producing up to about 50 m³ of leachate a day, a sequencing batch reactor (SBR) followed by a reed bed is a robust, low-skill option. The SBR removes organic carbon and ammonia in a small footprint, and the reed bed polishes the effluent. This article explains the proposal prepared for a landfill in Thailand that had no leachate treatment, and how an SBR cycle works.

The challenge: no treatment, and more leachate coming

The site was to host a waste-to-energy project, which was expected to increase leachate volumes. Design values were a maximum flow of 50 m³ a day and an influent COD of 1,500 mg/l. Treatment methods considered included evaporation, SBRs and reed beds or engineered wetlands. Reverse osmosis was deliberately not offered, because the concentrate it leaves is difficult to dispose of.

The SBR cycle

An SBR is an aerobic batch process: one volume of leachate is treated, then discharged. Because the bacteria stay in the reactor, a healthy sludge builds up. The cycle has six phases:

1. Fill. Leachate enters the reactor. With a single vessel the inflow must pause during the rest of the cycle.
2. React. Air is blown in at a rate that achieves the carbon and ammonia removal needed.
3. Settle. Aeration stops and sludge sinks.
4. Withdraw. Clear liquid is drawn from just below the surface, to avoid foam and above the sludge blanket.
5. Sludge removal. Excess sludge is taken from the base to keep the sludge balance right.
6. Stand-still. An optional rest before the next fill.

Which bacteria do the work

Heterotrophic bacteria oxidise organic carbon to carbon dioxide and water. Autotrophic bacteria carry out nitrification, converting ammonia to nitrite and then to nitrate. Where nitrogen removal is the main aim, nitrifiers dominate; where ammonia is absent they will not develop.

Why add a reed bed

A reed bed is planted with common reed (Phragmites). Reeds carry oxygen from their leaves to their roots, and leachate flowing past the root zone is cleaned by the microbes living there. It gives extra polishing and is a natural, low-energy stage. The SBR and reed bed pairing is described in the proposal as the workhorse of the leachate treatment industry.

Advantages and trade-offs

• Stable and simple. A landfill technician can be trained in a week or two.
• Low running cost. The main costs are energy and nutrient dosing.
• Small footprint for the SBR, though the reed bed needs land.
• Limited by strength. At 1,500 mg/l COD it is straightforward; stronger leachate with high ammonia may need pre-treatment such as stripping.

The offer was a turnkey package with a local civils contractor, with about six months from order to commissioning.

What to consider for your own project

1. Sample the leachate. COD, ammonia and variability decide the design.
2. Consider two vessels if continuous inflow is needed.
3. Plan sludge disposal.
4. Check consents. UK discharge limits are set through the environmental permit.

FAQ

What is an SBR?

A batch aerobic reactor that fills, reacts, settles and decants in sequence.

Do reed beds need a lot of land?

More than mechanical systems, which is why the SBR does the main work.

Why not reverse osmosis?

The concentrate left over is hard to manage.

Treating leachate at a small site?

Tell us your flow and COD.