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Detailed engineering of wastewater treatment plants coupling ozone and biotreatment

In order the well design the capacity of the ozone production needed in the final ozone plant, the main parameter to take in consideration was not the ratio g O3/g COT reduced, but the ozone dosage versus the COT reduction.

In such approach, we may consider with the effluent we will test on the final plant, that we will need A mg O3 per litre to remove it (usually around 3 g O3/g COT). In such conditions, and taking into account an ozone transfer of around 50% and with an effluent flow of B g/h, we will need 3xB g O3/h.

If we would let us the possibility to over dosage the ozone quantity introduced in the final plant, we could be able to reach a final COT which has given good results after the biological treatment. So, this over dosage will correspond to a max ozone capacity of 4xB O3/h.

From such analyses, we have designed for the final ozone pilot plant, an ozone generator able to produce 4.5xB g O3/h. After this first conclusion, we had to ask us the following question: Which type of feed gas do we have to use to produce that ozone quantity? In fact, there are two possibilities to produce ozone: From dry air or from dry oxygen. From dry air, we may produce ozone with concentrations between 20 to 50 gO3/Nm3 and from dry Oxygen, we may produce ozone with concentrations between 100 up to 200 g O3/Nm3.

How to select the best feed gas (air or oxygen): different parameters enter in consideration:
- Aim of the ozone treatment: oxidation, disinfection?
- Kinetic of reaction between ozone and COT reduction?
- Quantity of effluent flow to ozonate?

When we treat industrial waste effluents, the need of ozone is often important (which is the case in this application), and the effluent flow is, as a general rule, small. In our application, ozone is used as a strong oxidant, the kinetic of reaction is very low and the effluent flow will be also very low (one or two hundred liters).

In such conditions and in order to optimise the ratio ozone gas flow / water effluent flow, we will produce ozone from dry oxygen which will allow us to produce ozone with a low gas flow and high concentrations. It is the solution which has been retained for the final ozone plant and the ozone generator proposed will be able to produce an ozone concentration of 11,7% w/w.

About ozone contactor and ozone transfer, we have to take into consideration that the kinetics of oxidation of these wastewaters is usually low and the ozone transfer is bad with a lot of loose of ozone in the off gas. From these conclusions, we have decided to inject ozone in two contact columns in series.

As the effluent will contain a lot of salts and the ozone concentrations will be high, the different components of this pilot will be manufactured in Stainless Steel. Fron the ozone generator ozone gas produced from oxygen will be splitted in two directions : (i) one to an hyroinjector located upstream the bottom of the first ozone contact column, (ii) the other one to the porous diffusers located on the bottom of the second contact column. Each contact column will utilise a different mode of ozone transfer :
In the first contact column, we will have the possibility from a raw water pump to introduce a small quantity of ozone in an hydroinjector. Always in this first contact column, and in order to improve the contact between effluent and ozone gas, we have installed a recirculation pump which will suck from a second hydroinjector the ozone off gas leaving the second ozone contact column.

In the second contact column, we will use conventional ozone transfer by using porous diffusers made in Stainless Steel (classical porous diffusers in PVC are not compatible with the high ozone concentrations produced from ozone generator fed from dry oxygen).

In such a way, the ozone transfer will be optimized and the loose of ozone reduced to it maximum. The ozone contained in the off gas leaving the first contact column, will be destructed by a thermo catalytic ozone destructor.

With this pilot, we will be able to:
- Treat different effluent flows,
- Test different ozone concentrations and different ozone rates.
- Introduce different ozone quantities in order to feed the biological step with a pretreated effluent containing organic matters more biodegradable.

Ozone is an oxidant which may have a good efficiency on biorecalcitrant compounds oxidation but due to the high concentration of Femac and the associated high dose of ozone we have been obliged to introduce in the effluent, the ozone investment cost is high. The main possibility to reduce this investment costs, is to increase the ozone transfer in order to save a big part of the ozone loose.

Coupled with a biological treatment, ozone stays one of the cleanest technologies, which produces no sludges and no harmful products. In this case, biotreatment costs should be also taken into account.

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