Effluent Flow Distribution: New Developments.
Any approved waste water percolation area is designed to work well provided that hydraulic distribution of the effluent over the area is uniform. Part of this would be the assumption that distribution over the percolation pipes via a distribution box is even, as long as the box is 'installed with care to achieve a level position' (2009 EPA Code of Practice: Waste Water Treatment Systems for Single Houses). This would mean that these systems are heavily reliant on accurate installation. Off-level installation and moving soil are known as obvious contributors to a poor performance of any waste water treatment system.
Another known fact is that even distribution at low flow, which is typical for domestic systems, is difficult to achieve.This is confirmed by research carried out at Trinity College Dublin (Gill: 2005 and Patel:2008). Typically 70% oft he effluent coming from the septic tank would enter a distribution box at a flow lower than 0.5 litres per minute. The influence of the water’s surface tension in combination with its surface movement and other uncontrollable variables (e.g.a bur on a pipe or deposition of dirt) will have a strong influence on the route for the effluent.
New Test Results
An Environmental Science student from IT Sligo recently dedicated his thesis on this matter. A test rig, resembling a typical residential waste water treatment system,was built to facilitate his research. He found, quite remarkably, that slightly higher elevated outlets in a standard distribution box (due to tolerances) may well receive more effluent than the lower elevated outlets. See 'Test 1' in figure below.Outlets '1'and'6' appeared approx. 1mm. higher elevated than outlets '2'and'4'.
A closer look at these findings showed that as light change of slope of a connected discharge pipe played a rather unexpected role. On the distribution box, he tilted the entry of pipe 2 slightly downwards (without affecting the elevation) and the flow decreased. Outlets 4 and 5 were tilted to a slightly more horizontal position and the flows increased. See 'Test 2' in figure below. These findings might indicate why, over the years,many people have been failing to make a percolation field work effectively.
The student also tested a number of commercially available solutions from companies that obviously acknowledge the problem with a common distribution box. The test regime that was used is based upon EN12566, representing a typical daily load of approximately 600 litres. Access on the test facility was considered excellent and therefore not typically representing true practice in the field,which allowed every solution to be fitted and adjusted in a highly accurate manner.Tests were performed with the systems tilted under various angles to replicate field situations.
1 Adjustable Weirs
Adjustable weirs are cheap and easy to fit and their function is based on adjusting the outlet elevation. Despite accurate procedures however, all these solutions performed inconsistently. It was also found in all cases that at a slight 1° tilt of the distribution box, a number of the connected discharge pipes received no flow. It seems that these adjustable weirs may only provide some solution when a distribution box in the field is significantly tilted and that after fitting these weirs, the box should be frequently monitored.
2 Tipping Bucket Mechanism
An alternative solution that was put to the test was a pre-manufactured sample of a 'Tipping Bucket'. This unit is designed to deal with the sensitivity of distributing low flows. It collects a relatively large amount of effluent (approx. 0.7 l.) from the septic tank in one of the two sections of a tray-like tipping container that is placed on top of a hinge. When a certain point of unbalance is reached, the tray suddenly tips and discharges the full amount from that section into the outlets on one side of the distribution box. At this point, the other section of the tray starts filling and the process repeats itself, approx. 800 times per day. During the tests it was found that despite the robust principle, substantial amounts of effluent found its way into non-designated outlets after every discharge, particularly when the unit was tilted under a slight angle. This however was considered to be a matter of optimising the geometry.
3 Effluent Flow Splitter
This solution is based on yet another distribution principle. A prototype was made available to IT Sligo for testing. In this system the effluent is guided towards the centre of a convex shaped surface on which it subsequently spreads and runs into separate chambers.
Most remarkable fact was that even at 3° tilt, the unit distributed extremely uniform and consistent. Its compactness and simplicity seem to make it diverse applicable and it would be easy to retro- fit it into any existing manhole. Maintenance and cleaning seems an easy job.
Alternative solutions entering the market, like the tipping bucket and flow splitter, are an indication that constructive progress is currently being made in the field of effluent distribution devices.
From the review, it could be concluded that distribution based on level elevation of inverts (i.e. a straight forward distribution box,with or without adjusters) is principally flawed,performs poorly and inconsistently and needs to be abandoned, especially when better alternatives are available.