Hello fellow leather techies! It is some times depressing to hear from the preaching oriented about the perils of leather tanneries and how they poison the water. And we have an answer, some chemical guy from the neighbouring university is pulling some valves to put the tannery water through some pipes and pits to come with some numbers. If only we know how to at least talk about these to the outsiders it would be nice. Isn’t it!

I talked to a few of these engineers, broke my head against engineering diagrams ventured through terms like COD, DSS etc and came up with, what I hope is a simple version of what happens. Hope you like it.

The pollution

Normally clean water has about 8 mg per litre of oxygen dissolved in. We can drink that, fishes can breath and other buggies can thrive with that much amount of oxygen.

When some organic material is dissolved in water, the oxygen digests the matter and the water remains clean. If there is a chance of getting more oxygen, like in a large lake or running river or in a aerated fish tank, the oxygen can be replenished. Otherwise, the water can not support life like fish.

In tanneries we are processing hides and all the proteins (hair keratin, albumins, globulins, blood), veg tanning materials, phenolic retanning agents, some fats and so on eat up the oxygen in the water they are dissolved in. The quantity is expressed in terms of how much more oxygen is needed to oxidise them and make the water clean again.

So comes the term Oxygen Demand.

Tannery water also contains salts, acids or alkali. We will like out water to be neutral without some salty taste.

The last category is the metals like chromium and their compounds. The effluent treatment aims to remove all the impurities.

How?

1. The cheapest way to remove some foreign matter from water is filtering. By using a screen big pieces like, leather pieces, leather dust, shavings and other things are removed.
2. Since the processing in the tannery varies over the hours of the day, the water is collected in a large pit to obtain a uniform effluent.
3. Some chemicals such as sulfides and chromium are treated separately for effectiveness and their ability to adversely affect the treatment of organic wastes.
4. Some things precipitate. Lime is added to the mixed effluent to precipitate out some of these from the water. Then the water is pumped into another tank.
5. Some thins can be make to get together and settle down. By adding some polymers certain chemicals are made to settle down. Again the water from top is pumped off to the next stage.
6. Next stage is to put the water in a big tank and pump air into it. The air supplies oxygen to eat away a part of the impurities. Another part is munched away by specially introduced bacteria which make merry by eating, drinking and producing kids to do the same.
7. A major part of the pollutants are knocked by now. Water is now clean enough to let out into the river if the local authority is not hard nosed enough. But in many part of the world they are hard nosed. That is the problem now.