“Your product must have changed,” the site chemist told us in June. “We are using the same dose as February and the cake keeps failing.” It was not the product. It was the same kilogram-per-cubic-metre figure being applied to leachate that had spent eight weeks concentrating under a dry sky.
Almost every landfill runs two leachate chemistries a year without ever writing two schedules. The wet season brings volume and dilution; the dry season brings strength and stubbornness. One dose rate cannot cover both, and the transition is where most of the expensive mistakes happen.
Two curves that move in opposite directions
In a wet month, a site might handle 4,000 m³ at 1.8 kg/m³. In a dry month the same site handles 1,400 m³ at 3.1 kg/m³. Total polymer use falls by roughly 40 percent even though the dose rate nearly doubles, and the cost per cubic metre rises by around 70 percent. Finance sees a smaller invoice and asks why the treatment budget did not fall by the same amount. That conversation is easier if the two schedules exist as documents rather than as folklore.
The chemistry behind it is not mysterious. Less infiltration means less dilution of the same mass of salts, so conductivity, ammonia and chloride all climb. Evaporation from the lagoon surface concentrates further. A leachate at 12 mS/cm in March can be at 26 mS/cm in August, and the polymer has to work in a much more crowded solution.
Why the dry-season number looks worse than it is
Operators often read a rising dose rate as a failing product or a failing process. Usually it is neither. The kilogram figure is doing what it should: more ions and more dissolved organics compete for the same polymer chains, so more polymer is needed to reach the same free-liquid result.
What genuinely deteriorates in the dry season is the margin. A dose that was 15 percent above the minimum in February may be at the minimum in July, and any small variation in polymer lot, water temperature or mixing time pushes a batch over the line. The right response is not to chase the number upward indefinitely but to widen the safety margin on the first batch of each week and use it to set the rest.
Writing both schedules into one procedure
The workable format is one document with two columns and a switch rule. The columns hold dose rate, target batch size, mixing time and acceptance test. The switch rule is what matters: three triggers, any two of which move the site from wet to dry mode. Conductivity above a set point, thirty-day rainfall below a threshold, and sump level below a working minimum are enough for most sites, and all three are already recorded somewhere.
Put a date on the switch and a name next to it. A schedule that changes by consensus changes late, usually two weeks after the leachate has already turned. A schedule that changes when the chemist signs a line changes on the Monday after the trigger is met.
Stock planning follows the same split. The dry season needs less total tonnage but a higher proportion of the grade that performs in high-conductivity liquid, and it needs it in smaller, more frequent deliveries because the bags sit longer between batches. Ordering the wet-season volume into a dry-season store is how sites end up with caked pallets in September.
The June batch that failed twice
At the site that called us, the sequence was predictable in hindsight. Rainfall for May was 12 mm against a ten-year average of 70. Conductivity went from 14 to 24 mS/cm. The dose stayed at the February figure because nobody owned the change. The first failed batch was blamed on the polymer lot and a replacement was ordered. The second failed the same way, which is when the phone call happened.
A jar test at the current conductivity showed the dose needed to rise by about 45 percent. Nothing else changed. The site had spent three weeks and two rejected loads solving a problem that a monthly conductivity reading would have flagged before the first batch.
If you do not yet have a dry-season dose on file, the practical starting point is to take your current wet-season rate and test three points above it: plus 20, 35 and 50 percent. Run them on the same day, on the same leachate, and record which one clears the free-liquid test with margin. The polymer grade data will tell you what each product can realistically hold; the jar test tells you what your site needs this month.
Da maohua
Zhengzhou Flor New Materials Co., Ltd. specializes in SAP super‑absorbent polymer products. Serving personal hygiene, agriculture & horticulture, pet supplies and other industries, we leverage supply‑chain resources to provide particle‑size selection, performance matching and customized solutions based on clients’ practical requirements.
