Saturday, 13 October 2012

Papermill Effluent - Capacity and Turndown

Went to look at a paper-mill effluent treatment plant yesterday, which had never met its design performance, mostly down to two things - insufficient capacity, and lack of controllability. Of course if you have enough capacity, controllability is far less of an issue.

When I was working at Water Engineering in Banbury, someone told me that they had evaluated the margin of safety which went into their standard design procedure for aeration systems by comparing actual with theoretical performance and that it was on some measures as high as six. Some things were six times as large as they theoretically needed to be.

Experienced designers design plant not for a single duty point, but slightly beyond a design envelope, with enough controllable turn-down for the commissioning engineer to have a bit of capacity in hand.

Having been a commissioning engineer, I know that even on this basis, there are usually a few items which are supposedly comfortably oversized which end up being tuned to run under some operating conditions at the far end of their range.

I'm going to do a few sums to see if the plant as built can be made to work with minor mods, or whether something more radical is needed. My present feeling is that there are pipework modifications which will give a step-change improvement in plant performance.

Unusually the plant has been incredibly well monitored, and the wealth of data on present performance will give a solid baseline, allowing us to know for sure whether our changes have been effective.

Things have been less satisfactory with the Indian Restaurant job, though the owner has now realised that the supposedly septic tank friendly cleaning products he was sold are just the same as the old stuff. He has asked us to specify an alternative, though we sent him the list of EU eco-label products ages ago. We have suggested Ecover, though all of the eco-label products are equal as far as we know.

Hopefully if he starts using that, we might get somewhere. When we visited last week, the tanks were covered in a foot of foam, and several of the distributors which had been blocked by fat carried over by the cleaning chemicals had not been maintained.

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