3 replies to this topic

[niallmacdowell]

Hi,

I am currently involved in the design of some cooling towers for a bio-plant in Ireland. There is some debate regarding the source of the cooling water supply to the towers.

The local authorities have requested that we consider mains/municipal water supplies as a "scarce resource" and consequently we are looking at alternative options for the water supply. These are as follows:

1) River water, this water is hard and can deposit limescale. Also the river level varies quite a lot with seasonal changes, and the river levels falls significantly in the summer. As such, I believe that this is not a great choice for a water supply.

2) The outfall from the Municipal wastewater treatment facility-there is a significant quantity of this, but it is expected that this water will have a fairly high residual BOD-I believe the current EU legislation provides for a limit of 20ppm. I understand that this type of water is used in cooling towers in countries like Saudi Arabia where it is known as "grey water". It is my belief that this is not a good idea, as the risk for bio-growth is too high and the use of this water would require the addition of a significant quantity of biocides which would add further fouling potential.

3) There are some clean streams coming from the plant consisting of purified water and WFI streams that have been used for clean steam generation, CIP and manufacturing purposes. These are currently being sent to Chemical waste and Wastewater treatment facilities, which in turn dicharge to a sewer.

4) It might be possible to sink a well locally. The site is situated on/near an aquifer capable of producing a constant supply of approximately 150 cubic metres of water per hour, which would be more than sufficient for our requirements.

5) The final option available to us is the mains water supply, but as I said at the outset of this post, this is to be considered a last resort.

What I would like to hear is anybody who has any experience in cooling tower design with specific knowledge of the suitability of the various water supplies. If anyone has an opinion on which source would be most suitable I would like to hear from them.

Thank you

Niall Mac Dowell

Niall,

Here in Australia, where everything abounds except water, there is an increasing push towards re-use of outfall from local sewage treatment facilities. Usually (always???) it is given some RO treatment before being sent to industrial users. This is probably a little 'over the top' for the actual requirements of most industrial processes (the RO permeate is often better quality than local potable supply) but I believe it is done as a safety measure to almost completely ensure that no faecal-coliforms are sent to water users of any type (even though politicians and school-groups occasionally drink the non-RO water to demonstrate its safety!). This extra step obviously adds some expense to pre-treatment systems but also has many advantages.

1. RO water quality should be high enough to displace water other than cooling water
2. Can use this water to significantly reduce load on demin plants for boiler feeds and the like ie not just cooling..
3. As water is often better than potable supply it is often possible to use a blend of the RO water and water from your site treatment systems. This reduces fresh water needs and your sewer loadings at the same time.

You say that you have some "clean" water from the plant which is being sent to your waste treatment - it would seem that this would be the first choice......why pay to put it to sewer? of course mixing this with water from the local municipal treatment plant will decrease the BOD you are bringing in perhaps bringing it down to levels you find more aceptable. (Note that recycling of your wastewaters may significantly change the plant heat-balance and hence perhaps your cooling tower requirements.)

I'm sure you can get some more specific numbers on BOD limits by talking to cooling-tower and/or water treatment companies in your area. The water-chemical suppliers will have a good feel for the dosing and effects of their biocides. I cant see that these concerns would present too large a problem.

It sounds like you have a good opportunity for some good improvements for resource-efficiency improvements. Here in Aust. ther would be plenty of financial, political and water-availability pressure to help justify some really solid water re-use measures..... I unerstand that this wont be the case everywhere.

Good luck,

Nat.

[niallmacdowell]

Nat,

thanks for getting back to me-I appreciate your advice. Regarding your suggestion for the reuse of the clean streams, that "clean" water that is being discharged to sewer is considered to have too high a chemical content (it will have had some pre-treatment prior to discharge) for on-site reuse, and as a result it is being discharged to sewer.

I have a further question on the required pretreatment for cooling tower feedwater. In your reply you mentioned RO treatment, in my correspondance with people here in Ireland regarding pretreatment, they talk about using some carbon filtration and then sterilization with ozone, UV or chlorine. They seem to specifically reccomend chlorine due to its persistance.

Specifically my question is as following:

Should the water be sterile or simply clean? That is, what level of bio burden would be acceptable before the risk of (legionaires) desease becomes too great.

Would the RO treatment be enough to replace the subsequent treatment with ozone, UV or chlorine, or would both be necessary?

I have been able to get data regarding BOD and Phosphate levels in the outfall from the WWTP (~3 & 2 ppm respectively) and this doesnt seem to be a major issue for the vendors I have spoken to in that they are confident that they would be able to deal with that level of contamination. However as I have no information regarding the level of fecal contamination in the effluent this brings me back to the above point regarding the necessity of RO treatment.

Finally, do you know of anyone in Europe (or another non arid region) who successfully reuses WWTP effluent for cooling tower feedwater?

Thanks again for your help,

Niall

Niall,

Sorry for very late reply -

RO water will be microbe free and very clean but some further (persistent)treatment may well be necesary in cooling towre application due to the recirculating/concentrating nature of cooling tower systems but this would not be different to other clean sources.... it seems like you may have a fairly simple option just by using carbon and sterilisation as you menitoned... have you considered other contaminants inb the water though? WWTP water may have significant dissolved solids which may become an issue when cycled up (concentrated) in the CT (scaling, discharge limits etc)- depending on what exactly the useage is of course! The temperature of your recirculation water can also be a huge factor in the growth of legionairres and algae etc - as can light and other factors.

It sounds like the WWTP water is of reasonable quality - my guess is that any major issues you have will be regulatory rather than technical... it is quite possible/likely that you have a very specific reguation as to to whether recycled water can be used in a cooling towerand with what pre-treatment. If your local regulatoryautorities (for cooling towers , occupational health + safety, epa equivalent etc) are happy for you to use recycled water in this application then it sounds like you are on the right path.

I mentioned RO largely because of the opportunities it gives for recycling your own contaminated waste water and because it tends to be a prerequisite for municipal WWTP re-use in Australia .... whilst it provides very clean water it may not be the best option if there is another source which does not require further removal of dissolved or other non-microbial contaminants... there may be a considerable cost in plant, energy, membranes etc but also considerable benefits in water savings, effluent reduction. Water may sometimes have to be pre-filtered prior to RO to remove any suspended material.

I dont want to give any advice that is too specific because my background has been more in identifying an d exploring opportunities for water/materials and energy re-use/savings and so cannot claim to be a cooling tower or legionairres expert. I have been involved in enough re-use projects howver to have seen that there are few technical in the reuse of water given current technology. I dont have specific no-arid examples as you requested but I do note that the US EPA adn others all around the world are encouraging exactly this sort of project - as well as Australian authorities (see online). An Australian example of large scale re-use is the Gippsland Water Factory in SE Aust, where water is to be taken from several sewage tratment plants abd given tertiary treatment for use by industry (largely for pulp and paper). This major project is in its earlyto mid stages and there is limited detailed info online but it show that it can be done and is done with success. Any resource saving opportunities are thererfore well worth persuing.

I hope this helps and your project goes well,

Nat.