A Design for Laterals

A contractor asks for comments on an orifice-down design for piping in drainfields with pressure distribution networks

Question:

Before new rules were issued in British Columbia, it was common practice in my area to design a distribution network with the laterals facing down. We would build the system with the orifices facing up, and after the pressure test, rotate the laterals 180 degrees and cover with more rock.

I always liked this method. There is no need for extra drain holes because it is obviously free draining, you save some money and installation time on installing shields, and — this is my observation only — the distribution is better, because the effluent hits the nearest drain rock and is diverted sideways. This does a nice job of breaking up the spray and distributing it farther than it can go under a small orifice shield.

I have walked up and down many networks set up this way, before covering, and I have never seen an orifice that seemed restricted in its flow. This method also eliminates the need, at least in our warm climate, for a weep hole in the manifold to ensure that the network drains.

I have always thought this was a counterproductive feature in a system, as you are introducing the effluent at a lower separation distance, and quite a lot of it, as after the pump shuts off, more of the network’s remaining effluent will drain to this low spot. I am just looking for input. Is anyone else using this method?

Answers:

We prefer the holes down in our pressure distribution networks for the same reasons you have mentioned, perhaps with the exception of the argument of better distribution. I have a colleague who prefers the holes up, and he has his reasons.

My concern is potential freezing if water stands in the pipe between doses. He tries to design his systems so that the pipe drains back to the tank. I don’t think either one of us would win a debate with any significant evidence as to which is better.

We test them with one orifice in a cap on the flush assembly at the end of each lateral. Then the cap is replaced with a solid cap after testing. This works well. You don’t really need to see the discharge out of every hole.

To be more accurate, you could use a standpipe on the flush fitting on the end of the lateral, rather than an orifice. The orifice is easier. We have probably 30 years of experience with hundreds of these types of installations with very few problems — almost none — and those are not related to whether the holes were down or up.

And, we do have a company that provides maintenance services on onsite systems, so we do get out of the office to monitor our designs to a significant extent.

I’ve seen pressure distribution done in a number of states in the southeastern United States, and my recommendation would be to go with the design employed in North Carolina. It’s the most robust method I have found.

You are correct as far as you go, but you still need at least one up-facing orifice at the distal end to allow the air to bleed out of the lateral on pressurization, and you will find in time that it will overload the manifold end of the lateral because the orifices closer to the manifold will start to discharge well before those at the distal end on every cycle.

It certainly will depend on several design choices made, but we have not found either of those problems in our system designs. In fact, I believe that the water tends to rush toward the distal end rather than hang around the orifices near the manifold. (I can’t prove it because I’ve never been in there.)

But in our designs we typically use 1/8-inch-diameter holes and design for residual heads in the range of five to 10 feet. If you use larger holes with less head, you may find the problem described above.

As a side point, always design the laterals with cleanouts so that they can be flushed on a regular basis. You will find that solids tend to build up in the ends of the laterals over time and need to be flushed out or they will clog the last hole. This is due to the slow velocity in the lateral ends.

Then the second-from-last hole becomes the last, and it will eventually clog, working backwards toward the manifold, and overloading the inlet side of the system. A neglected system will eventually show signs of surfacing near the inlet.

Whether the orifices are faced up or down, there is a somewhat higher volume discharged at the manifold end. During pump startup, if the force main drains back to the tank, it can be more significant. But if there is a check valve in the tank, then the system remains charged and distribution is more uniform. With the orifices faced down, the effluent essentially “falls down” through the nearest orifice.

On the issue of cleanouts, they are important. As long as the lines are flushed regularly, it doesn’t really matter what they look like, but if maintenance is neglected, as it often is, then it is good to have a sweep design so they can be mechanically cleaned.

I have flushed systems that are not very old and have found several feet of sludge at the ends of the laterals. I also have a few systems with poor cleanout design and maintenance, and they are now partially plugged. It’s a problem that doesn’t get much press for some reason.



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