A crust or grease mat has formed on the surface

How a mat forms

Fats, oils and grease arrive at the lagoon as an emulsion or as free product, and in either case they are less dense than water. In a lagoon with enough treatment capability they are degraded on roughly the timescale they arrive. Where arrival outpaces degradation, the surplus floats, and floating material does not stay dispersed — wind pushes it downwind, it consolidates, and it collects rags, plastics and settled debris as it goes.

The result behaves as a solid. On a dairy or food processing lagoon it can reach several inches thick across a large share of the surface, and by that point it is no longer an aesthetic complaint. It is a treatment problem.

Why it costs more than it looks

• Oxygen transfer stops across the covered area. A lagoon relies on surface exchange and, in a facultative cell, on photosynthetic oxygen produced in the upper layer. A mat removes both.
• The cell beneath turns anaerobic, which changes what happens to material settling into it and slows digestion of the blanket.
• Working volume falls. A mat several inches thick across most of a cell displaces capacity you have already paid to build.
• Everything mechanical gets harder. Pumping, mixing, land application and any eventual dredging are all slower and more expensive with a mat in the way.

What degrades and what does not

This is the distinction that decides whether treatment is worth ordering. Fats, oils and grease are organic and biodegradable. Lipase is one of the three enzyme classes that does the work in this category, and degrading FOG sits squarely within what these organisms do to material.

Rags, wipes, plastics, grit and mineral debris are not biodegradable. Where a mat is substantially bound together by that material — and an established one usually is, to some degree — treatment reduces the organic share and leaves the rest floating. An operator expecting a clear surface from a product that can only act on part of what is up there will conclude it failed, when in fact it did what it can do.

The upstream question worth asking first

A mat is a symptom of FOG arriving faster than it degrades, and treatment addresses one side of that. If the arrival rate is what changed — a new industrial connection, a failing grease interceptor upstream, a receiving programme without limits — then treating the lagoon repeatedly is paying to manage a problem with a cheaper fix at its source.

That is not an argument against treatment. It is an argument for knowing which of the two you are buying, because a lagoon whose FOG loading has genuinely risen needs treatment as an ongoing programme rather than a one-off, and the budget for those is different.

What to expect from an order

Both dry products act on organic material, and which one fits depends on where the material sits rather than what it is called. A surface mat with a healthy water column beneath it is a different proposition from a mat sitting over an established sludge blanket, and the second usually means addressing both. The planner will say which format fits the lagoon you describe, and will say plainly when the answer is that biological treatment is not the right spend.