The COD-to-Ammonia Ratio Sets the Dose, Not the COD Number Alone

Two leachates were tested the same week, both quoted at COD 18,000 mg/L. The first came from a cell five years old, ran at pH 5.8 with ammonia at 420 mg/L and a BOD to COD ratio of 0.48, and gelled firmly at 14 kg/m³. The second came from a closed cell fourteen years old, ran at pH 8.2 with ammonia at 2,300 mg/L, and needed 26 kg/m³ to reach the same firmness. Same COD, nearly double the dose: a quote compared on COD alone backs the wrong one.

This is the single most useful screening relationship in leachate solidification, and it is routinely ignored because COD is the number that appears first on every lab report. This article sets out why the ratio of COD to ammonia nitrogen predicts dose behaviour better than either figure alone, how to build a two-point dose curve from it, and where the rule breaks down.

Why Young Leachate Gels More Easily

Acid-phase leachate is a dilute organic soup. Its ionic strength is usually below 0.1 mol/L, its ammonium content is low, and its dissolved solids are dominated by volatile fatty acids that carry little charge. An anionic polyacrylate chain in that environment stays extended, its carboxylate groups repel each other, and the network opens up to hold water. Absorption in the 45 to 60 g/g range is normal.

Methanogenic leachate is a brine by comparison. Ammonia at 2,000 mg/L and above, bicarbonate at 8,000 to 12,000 mg/L, chloride from co-disposed waste and sodium from the waste itself push ionic strength to 0.3 mol/L or higher. The counter-ions screen the fixed charges on the polymer, the chain coils, and the network that was open in the young leachate closes down. Absorption falls to 20 to 30 g/g. The COD in that liquid is still high, but much of it is recalcitrant humic material that does not gel and does not interfere either. It is the salt load, tracked by ammonia, that does the damage.

Using the Ratio as a Screening Number

Divide COD by ammonia nitrogen, both in mg/L, and read the result as an indicator of where the leachate sits in its life. Above 20, you are dealing with an acid-phase or young leachate and should expect doses in the 10 to 16 kg/m³ band. Between 8 and 20, expect 16 to 22 kg/m³. Below 8, expect 22 to 30 kg/m³ and check chloride before you commit. These bands assume a standard 300 to 400 g/g free swell grade and a target that passes a paint filter; they shift with the polymer and with the acceptance criteria.

The ratio also tells you what to measure next. A low ratio with moderate conductivity points to ammonia as the main interferent, which responds to dilution with fresher leachate. A low ratio with conductivity above 30 mS/cm points to chloride, which does not respond to dilution at all and has to be handled by dose or by grade.

Build a Two-Point Curve Instead of One Number

Most sites test one sample and fix one dose. That works until the cell turns over, and then it fails quietly for a month before anyone notices the rejection rate climbing. A two-point curve costs one extra hour in the lab and covers the transition. Take a sample from the oldest cell and one from the newest, run five doses each in 500 mL jars, and plot gel firmness against dose. You will get two curves that converge at the top end and diverge at the bottom, and the gap between them is your seasonal operating range.

Set the production dose at the knee of the older curve plus 10 percent, and set a trigger for re-testing when the ratio moves by more than 30 percent or conductivity by more than 5 mS/cm. On a site with three active cells and a shared sump, that trigger fires two to four times a year and is far cheaper than a monthly full panel.

Where the Ratio Misleads You

Four situations break the relationship. Industrial co-disposal with high sulphate produces a leachate with moderate COD and very high ionic strength, so the ratio looks healthy while the dose behaves like a closed cell. High-calcium leachate from construction and demolition waste crosslinks the polymer at the particle surface and produces a crusted gel with a wet core, which reads as a dose failure but is really a mixing failure. Sites receiving liquid waste imports can swing by a factor of two within a week. And any leachate that has been through reverse osmosis is a concentrate, not a leachate, and needs its own curve entirely.

Used with those limits in mind, the ratio is the cheapest early warning a landfill has. Two numbers off a standard lab report tell you whether to expect a 14 kg/m³ day or a 26 kg/m³ day, which is the difference between a load that ships and a load that comes back. The SAP powders we supply for leachate work are tested in both dilute and saline leachate, so you can see how far a grade falls before you commit to a season of it.