Before You Buy Methanol, Divide Two Numbers

If your nitrate is failing, somebody will eventually offer to sell you a carbon source — methanol, molasses, or a proprietary blend with a name and a data sheet.

Before you agree to that, divide two numbers you probably already have.

What destroying nitrate actually needs

Nitrate is where ammonia ends up. Give the plant plenty of oxygen and the ammonia converts through to nitrate, which is why a plant that has just solved its ammonia problem sometimes finds it has bought a nitrate problem.

Getting rid of nitrate is the opposite operation. You take the air away in a tank — an anoxic tank — and bacteria strip the oxygen out of the nitrate molecule instead, releasing harmless nitrogen gas to atmosphere. That is the same reaction that will happen uninvited in a clarifier if the sludge blanket sits too long, which is one of the three quite different things a jar test tells apart.

To do that they need something to eat. Organic strength. Carbon.

Roughly four parts BOD to one part nitrogen. That is the ratio worth memorising.

So compare your own two numbers

Take the BOD of your incoming wastewater and the total nitrogen of your incoming wastewater, and divide.

If the answer is comfortably above 4, your wastewater already carries the food needed to destroy its own nitrogen, and you should not be buying carbon at all.

On a poultry dressing plant we worked on, that ratio came out at 14.4 to 1 — more than three times what denitrification needed. The wastewater was more than capable of dealing with its own nitrogen. Purchased carbon on that site would have run to several hundred thousand pesos a year, every year, to solve a problem the water was already able to solve for free.

What it needed was not a chemical. It was a tank with the air turned off, in the right place, fed by the water it already had.

The awkward part is that most people cannot do this division, because they do not have an influent result to divide. That is not a criticism — we ask for the influent analysis on every job and almost nobody has one, so we assume, and we assume safely, which means bigger and dearer.

The two bonuses nobody mentions

We often install an anoxic stage even where nitrate is not the failing parameter, for two reasons that have nothing to do with nitrate.

It gives you back alkalinity. Converting ammonia is an acid-forming process — it eats about 7.1 mg of alkalinity for every 1 mg of ammonia nitrogen, and when the buffer runs out the pH falls and nitrification slows down and stops. Destroying nitrate hands roughly half of that alkalinity back. On plenty of plants that is the difference between dosing lime or soda ash every single day and not dosing at all.

It costs almost nothing to run. A tank with no air in it uses no blower power. You are moving water and letting biology work.

That second point is why, on plants where the nitrate limit is not under threat, we still build the pipework and install the pump — and still run a return at about one times the incoming flow. Not to chase nitrate. Purely to bring alkalinity back to the front of the plant.

Where nitrate genuinely does have to come down, we typically run the return at three to four times flow, sometimes two. Which one is right depends on the plant, and we test rather than assume. There is more on how we handle nitrate as a job in itself, and on which nitrate limit applies to your discharge — it is one of the parameters that rarely fails first, and it is worth knowing where you actually stand against it.

What this means when you are reading a quotation

If a proposal includes a recurring chemical line for denitrification carbon, ask the supplier what your BOD to nitrogen ratio is. It is a fair question and it has one right answer, taken from your own laboratory results.

If they cannot tell you, they have not looked. If they can tell you and the number is above 4, ask why you are being sold carbon your wastewater is already supplying.

An ongoing chemical cost is the most expensive thing in a treatment plant over its life — far more than the steel. It is worth ten minutes of arithmetic before you commit to it. This is one of the questions we would ask before signing anything. If you are still at the stage of working out what the plant itself should cost, our STP price estimator will give you an indicative figure from your flow.

Your wastewater is not that dressing plant's wastewater. A domestic building with weak sewage can genuinely be short of carbon and may need a supplement. The point is that it should be established from your numbers, not assumed.

Send us your influent BOD and total nitrogen and we will do the division with you. Book a free 15-minute consultation.