Leachate recirculation and what it does to solids loading

Recirculation is sold as a volume reduction measure, and on the flow meter it works. Send the leachate back over the waste mass and less of it leaves the site. What the flow meter does not show is that every litre returned comes back stronger, warmer and carrying more dissolved solids, and the polymer bill per cubic metre climbs for years.

That trade is not automatically bad. It is simply not free, and the sites that handle it well are the ones that measured the second half of the equation before they committed.

What happens inside the waste mass

Recirculated leachate wets waste that would otherwise stay dry, which accelerates hydrolysis and acidogenesis. Moisture content rises, settlement increases, and the waste mass moves through its decomposition stages faster than it would under natural infiltration. In principle this shortens the long-term emissions curve.

The immediate effect is a stronger liquor. Total dissolved solids climb, chloride accumulates because it has nowhere to go, and ammonia keeps rising as proteins break down. Calcium and magnesium rise too, as the acidic front dissolves carbonate material. After two or three years of recirculation, a site that started at 12 mS/cm often sits above 25.

Solids loading is the part that bites

Higher TDS means the gel has less room to swell. But the bigger change is in the solids: recirculation mobilises fines, carries sludge from the sump back over the waste, and produces a leachate with a much higher and much finer suspended solids load than a site without recirculation.

Fine solids do two things to a batch. They increase the polymer demand because the chains have to bridge many more particles, and they make the cake wetter because the fines hold water in the interstitial spaces. A site can find itself using 30 percent more polymer to produce a cake that is 4 percentage points wetter. Both effects push cost per tonne of dry solids up at the same time.

What to measure before and after

Four numbers tell the story: TDS, suspended solids, calcium and sulphide. Track them monthly against a baseline taken before recirculation started. If TDS is climbing faster than the modelled concentration factor, the mass balance is telling you that the waste is generating more than it is retaining, and the recirculation rate may be too high.

Also track polymer consumption per tonne of dry solids rather than per cubic metre. Recirculation reduces cubic metres, so the per-cubic-metre number looks better while the real cost rises. Per dry tonne is the only figure that survives a change in volume.

Operating rules when you recirculate

First, bypass recirculation when the leachate is at its strongest. A batch at 35 mS/cm does not need to go back over the waste and come out worse; it needs to leave the site. Second, cap the recirculation rate at something the waste mass can actually absorb, because ponding on the cell surface produces a different set of problems.

Third, build a solids removal step before the pad. A settling tank or a simple screen removes enough fines to bring the dose back into a predictable range. Sites that skip this step end up filtering at the mixer, which is the most expensive place to do it.

The site that recirculated into a corner

A 300,000 tonne site began recirculating in year three and cut tanker movements by 45 percent, which looked like a clear win. By year six the leachate was at 41 mS/cm, the polymer dose had tripled, and the cake was failing load tests because of free liquid. The recirculation was turned down, but the TDS did not fall; the salts were already in the waste mass and kept coming out.

The recovery took two years and a change of route: the strongest fraction went to a thermal treatment contract, and only the weaker flush water was recirculated. Total cost ended lower than the all-tanker alternative, but higher than the original projection, which had assumed constant leachate strength.

If recirculation is on your site or being considered, ask for the strength projection over ten years, not the volume projection over two. The high-ionic-strength SAP grades hold up better in concentrated liquor, but no grade makes a 40 mS/cm stream behave like a 12 mS/cm one.