Lagoon turnover after an algal bloom: what it does to your mix

A green lagoon looks like a healthy pond to anyone who has not spent a summer watching one. The colour is algae, and the day it turns over, everything that settled under that green layer comes back up into the water you are about to dose.

Turnover is not a rare event. It happens after a still, hot spell, after a sudden cold night, or after a heavy rain that pushes cold water across a warm surface. The surface layer sinks, the bottom layer rises, and the leachate arriving at your mixing pad is a different liquid from the one you tested on Friday.

What comes up with the bottom water

Three things matter for solidification. First, sulphide. Anaerobic bottom sediments generate it steadily, and it sits there until the water column mixes. Sulphide reacts with the polymer’s available sites and with any iron you have in the liquid, so a batch that needed 2.2 kg/m³ can suddenly need 3.0 and still look under-dosed.

Second, fine biomass and dead algal cells. These are not heavy solids that settle out in the mixer; they stay in suspension and hold water in the cake. You get a batch that sets, passes a hand squeeze, and then bleeds a thin brown liquid an hour later on the trailer.

Third, a pH swing. A bloom can push daytime pH to 9.5 or higher through photosynthesis, then drop several tenths overnight when respiration takes over. If your polymer is sensitive to pH at the wetting stage, the same dose behaves differently at 07:00 and at 15:00.

Why the first batch after turnover usually disappoints

Most sites respond to a colour change by adding polymer. That helps, but it treats the symptom. The bigger issue is that the batch is being asked to do two jobs: neutralise a reducing, sulphidic liquid and bind water. Sulphide consumes oxidant and polymer alike, and the oxygen demand of the mixed liquid can be high enough that the gel never reaches its full swell.

A cheap pre-check changes the outcome. Take a litre, add a few drops of hydrogen peroxide or aerate it for ten minutes, then run a jar test against an untreated sample. If the pre-treated sample needs noticeably less polymer, you are dealing with a reduced liquid and the answer is aeration or a pre-oxidation step, not more powder.

Dosing through a turnover week

Split the day into smaller batches and test more often. A 20 m³ batch that fails costs you twenty minutes; a 60 m³ batch that fails costs the shift. Four-hourly checks on pH, sulphide and conductivity during a turnover week are not excessive, because the liquid can change measurably within a single shift.

Skim what floats before it reaches the mixer. Algal mats and scum carry a lot of water and very little solids, and they are the fraction most likely to produce a wet, unstable cake. A simple surface draw-off or a coarse screen at the sump inlet removes enough of it to make the dose predictable again.

Keep the lagoon level stable if you can. A turnover driven by a sudden inflow of cold rainwater is worse when the lagoon is drawn down and the bottom sediments are close to the surface. Where the site has storage, holding the level through the event and dosing from a partly full lagoon gives the water column time to re-stratify.

The lagoon that turned in a single night

One site we worked with had a 9,000 m³ lagoon that had been stable and green all summer. A cold front in late August dropped the air temperature 11 °C in a night. By morning the lagoon was grey-brown and smelled of sulphide from thirty metres away. The first three batches that day foamed in the mixer, and the fourth set into a cake that sweated brown liquid in the trailer.

The site had no pre-oxidation step and no spare aeration. What rescued the week was a temporary cascade: pumping from the lagoon over a short weir into the dosing tank, which added enough oxygen to knock the sulphide back. Dose fell from 3.4 to 2.6 kg/m³ over two days as the water re-stabilised.

If your lagoon has a history of blooms, the time to plan for turnover is in the dry season, not the morning it happens. Start with a sulphide and pH reading on the same sample you use for dose testing, and keep a jar of peroxide in the test kit. The solidification field guide covers the batch sequence for reduced liquors, including where a pre-oxidation step fits into a normal shift.