The Theology of Sewage Works

My project reviewer commended my ‘Capacity Statement’ [A statement of the hydraulic capacity of the seven feed pipes for a complicated bank of 24 filters at a sewage works]


He said it was ‘very professional’ and ‘inspired trust’.

This was mainly based on the opening paragraphs, which I quote for you below with certain ‘fudge’ words highlighted.

Statement of Capacity of Existing Filter Feed System


July 2012


Selection of calibration models


The calibration exercise considered four flow survey cases and produced up to three possible calibrated models for each of the individual lines corresponding to relevant flow survey cases.


Uncertainties regarding flow measurement at the site resulted in a need to review the possible capacities for ‘low’ and ‘high’ flow measurement interpretations.


A pragmatically selected collection of the more realistic calibration models for the individual lines (while still reflecting a conservative approach particularly with respect to flow measurement) was combined into a system model which was then tested across a range of flows up to FTFT [the maximum]. Variant models were made and tested for different flow split design cases.

If you read between the lines of the above, you will perceive that we haven’t really got much clue as to what is going on.

We went to the site for four separate flow surveys in different conditions. They were fairly dry days and there simply wasn’t enough water to test all of the pipelines each time. Then we found out that the site’s main flow meter had broken down and we didn’t really know what we were testing with at all. We spent moths trying to match the contradictory surveys to each other and trying to build a believable computer model of the system – one where the model predictions would match the realities we observed on the site so that we could build future designs on. Some of the more extreme conclusions implied that the site’s original designers had been so totally incompetent as to only get half of the design flow through it. My reviewer said that this could not be true, and we should not include that particular survey result in our conclusions. Even with the more credible surveys, the results were contradictory. We think that some of the outlet pipes are too close to the weirs so that air bubbles are being sucked in which then accumulate in the pipe and reduce its effective size. You would have a different amount of bubble each day, hence the variation in survey results.

The scientific answer would be to spend years on site surveying every possible parameter, spending £xx,000 on equipment, and interfering with the operation of the site as we tweak the valves to test all different options. The reality is that you can’t do that. The Environment Agency is likely to prosecute the water company if the site’s problems are not resolved within the current Asset Management Plan, which ends in 2014 and it takes at least two years to implement a solution. We have to do it now.

Science is limited – by time and budget - in its ability to answer the question. We have to make a judgement call, and write a statement of capacity that gives a reasonable degree of confidence in the results without hiding [too much] the considerable uncertainties we have in it.

It’s not perfect, but its as good as its ever going to be, and based on that limited information we have to make a decision as too how to proceed.

Don’t worry – it’s only a £20million project that we are exercising this guesswork on!



So my capacity report I use words like ‘Uncertainty’, ‘Interpretation’, ‘Conservative’ (ie guessing on the safe side), ‘Pragmatic’.

This is like our faith, isn’t it?

In our life we see a lot of evidence that Christianity is true.

But a lot of that evidence is incomplete. Muddled. Contradictory. Based on assumptions. Based on data that turns out to be incorrect. And yet on this incomplete data we have to make a decision – reject or accept the message.

Now, if this level of evidence is sufficient for my water company client to spend £20million, then I don’t think its unreasonable for a similar level of evidence to allow us as individuals to decide in favour of Christianity.

We can’t prove it scientifically. But it is sufficiently reasonable to proceed, cautiously, and build our lives on it. And we have to preach with a reasonable degree of confidence that we are right but also with honesty about the uncertainty.  Hopefully such a presentation of the gospel will be ‘very professional’ and ‘inspire trust’