If your landfill leachate is normally within its discharge limit but suffers occasional peaks of suspended solids, a two-stage filter is usually a better fit than a sophisticated flotation plant. In a temporary treatment scheme for a UK landfill, the preferred answer was a vortex filter for roughing, followed by a granular (sand) filter for polishing. This post walks through the five options that were considered and why four of them were set aside.

The challenge: random peaks, not a steady load

The landfill operator needed a temporary leachate treatment plant mainly to reduce suspended solids. Most of the time the leachate was within the discharge limit. Occasionally, though, solids peaked at up to 1,200 mg/l. The likely causes were a disturbance of some sort, either site operations or a heavy rainfall event.

That pattern matters. A plant designed for a steady high solids load will be oversized and expensive to run for most of the year. A plant designed for the average will fail on the day it matters. The right answer is a filtration stage that can cope with random flushes and does little work the rest of the time.

The approach: five options compared

1. Dissolved air flotation

Dissolved air flotation (DAF) was judged too sophisticated for the level of cleaning required. It would add excess energy cost and could reduce solids to a lower level than necessary, which means more sludge to handle on site.

2. Vortex filtration

A vortex filter accelerates the liquid around a tapered conical drum, which removes particulates. Its useful feature here is control: the degree of filtration is set by how much of the flow passes through the vortex. It can be sized for the maximum flow, and when solids are low it can be partly or completely bypassed.

3. Granular bed filtration

Sand filters are simple to operate and relatively low cost. They are normally installed as two beds on a duty cycle, with one on line while the other is back-flushed. They also remove some BOD associated with solids and a certain amount of colour. The risk is blocking: excessive build-up of other solids can foul a bed quickly, so pilot-plant trials would be needed.

4. Clarifier filtration

A clarifier is perhaps the simplest option, but also carries the most uncertainty. Settlement depends on particle size and density, which vary through the year with the source of the particles. Pilot trials would be needed to size it properly.

5. Screen filtration

Screens have one practical advantage: they can be changed over if the nature of the solids changes with time. Again, further site investigation and possibly pilot trials would be required to understand what has to be removed.

Why a vortex filter followed by a sand filter

The preferred combination was a vortex filter for roughing, followed by a granular filter for polishing. The logic is sequential:

• The vortex stage takes out the bulk of a peak load, and can be bypassed when the leachate is already clean.
• The sand filter then polishes what remains, without being overloaded by a surge.
• All removed solids are returned to the landfill, so no new waste route is created.

Approximate dimensions were given for both units. The vortex filter would be about 1.5 m in diameter and 4 m high, built from 304 stainless steel. The granular filter would be about 3 m in diameter and 4 m high, and would include the pumps and controls needed to run correctly.

The caveat

The offer was explicitly subject to further analysis of the leachate and confirmation that the treatment objectives had been correctly identified. That is the honest position on any leachate scheme. Leachate composition varies from site to site and season to season, and a filter selected on assumed solids characteristics can fail on real ones.

What to consider for your own project

1. Characterise the peaks. Know how high solids rise, how often, and what triggers them.
2. Match the process to the duty. If the leachate is usually compliant, a bypassable stage avoids paying running costs for treatment you do not need.
3. Plan for the solids. Whatever you remove has to go somewhere. Returning it to the landfill is one answer, if your permit allows it.
4. Consider pilot trials where a sand filter, clarifier or screen is being selected, because particle characteristics drive performance.
5. Check your consent. Discharge limits are set in your environmental permit or consent. The Environment Agency explains the landfill permitting framework in its guidance for landfill operators.

FAQ

What causes suspended solids peaks in leachate?

In this case the likely causes were disturbance linked to site operations or a heavy rainfall event.

Why not use dissolved air flotation?

It was considered too sophisticated for the cleaning required, with higher energy costs, and it could reduce solids further than needed, leading to more sludge handling.

Can a vortex filter be bypassed?

Yes. It is designed for the maximum flow loading and can be partially or completely bypassed when solids are low.

Why use two sand filters?

Two beds run on a duty cycle, so one can be back-flushed while the other stays in service.

Dealing with variable leachate?

Send us your leachate analysis and discharge limits and we will suggest a staged treatment approach. See also our post on sequencing batch reactors for ammonia and COD.