New Improved Holding Tank

[Homer Shannon]

Last fall I posted this extensive discussion on Crusiers Forum; http://www.cruisersforum.com/forums/...ly-111522.html. The gist of this was that my macerator was not working reliably and my whole septic tank set-up was suspect. Over the fall and winter I engaged in a complete rebuild of the system. This has resulted in a system that should be trouble free and, should it give me trouble, is highly accessible for monitoring and repairs.

A key component to the new system was a new holding tank. Our boat, a 1978 Bristol 29.9, was not built with a holding tank. One of the two first owners equipped it with a twenty gallon, home-made, tank built out of fiber glassed plywood. The tank is under the port-side v-berth and has never leaked, though we do get some septic odors from time to time. Inspections have shown the tank to be rot free, but still it has always worried me. With all the other issues at hand, replacing the tank was a priority.

Other desired changes to the system included: re-locating the macerator pump to a location where it could be worked on easily, separating the deck plate and through hull discharges and moving all the fittings to the top of the tank – no possible points of leakage.

I searched for a production tank that would fit my location, capacity and budget. I was figuring on about $150 to $300 based on what I had seen in a number of marine catalogs. I was disappointed when I discovered that there wasn’t a single tank that would meet my specifications for size, shape and capacity. Between Jabsco, Trionic and Ronco, there must be five hundred tank sizes available. The only one that would fit and provide me the vertical clearance I wanted to allow mounting the macerator pump on the top of the tank was only fourteen gallons and it cost a whopping $400. I decided to take a different approach.

Somewhere in all my web searching I uncovered the topic of plastic welding. Honestly, I had never heard of plastic welding before, but it seemed like an interesting concept. I viewed a few Youtube videos and did some more research. This is not overly complicated. I proceeded to learn that both HDPE (High Density Polyethylene) PP (Polypropylene) are suitable for waste tank construction. They are also the two easiest materials to weld and they are sold at Grainger in varying thicknesses and sizes up to 48” x 96”. Both materials are easy to work with and can be cut and shaped using traditional work working tools – with some cautions and adjustments. Armed with this new knowledge, I headed back to the drawing board and reconsidered my project.

I came up with a design that would fit in the available space, have a capacity of 18 gallons, would include all the top-mounting and other features I desired, and most importantly, be constructable using standard plastic welding tools in the hands of a rank novice plastic welder. After finessing this design a bit further I started ordering my materials:

- Plastic Welder 96712 from Harbor Freight – not a great tool, the first one failed in ten minutes, but reasonably priced at about $70 with a good warranty — the second one is still working.
- Speed Tip from HEJET — expensive at $80, but it makes beautiful welds quickly.
- Polypropylene welding rod from HEJET.
- 3/8” Polypropylene sheeting from Granger — in hindsight HDPE might have been better, but the PP will be fine. Also, 3/8 is probably thicker than necessary.
- Various fittings from FlexPVC and Grainger
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Cutting the PP sheets was no more difficult than cutting plywood parts in the same size and shape. You need a good carbide blade for your saw and a radial arm saw would have been useful. My shop is limited to a table saw, rotary hand saw and a chop saw (which I did not use for this project). After cutting out the main pieces, I used a router with a 45-degree bit to chamfer the edges were I would be welding.

I made a few trial welds on some small bits; there isn’t a big learning curve to it. Pretty much watch the Youtube videos and take your time. The Harbor Freight welder does a good job despite its price, which is a quarter of the price of its competition. Take care to note how they instruct you to shut it down, however. Failure to follow this rule will result in the machine failing. You will want to wear some ear plugs. The machine is noisy – like a loud hairdryer – and it is stressful to work with without ear protection.

The analogy of a hair dryer is pretty good for the welder. It is sort of hopped up hair dryer; it has a heating element, a blower, and tip that concentrates the hot hair… really hot— up to 800 degrees! Welding the plastic parts requires that you heat the parts and apply the welding rod to the heated surfaces. This can be done by the “pendulum weld” technique, using the tips supplied with the tool, or for long runs, you can use the speed tip that lays down a long continuous bead of weld. I used both depending upon the locations within the tank.

An issue with the plastic welding that I did not anticipate, is that the plastic sheets tend to warp as you work on them, even though you are only heating a small portion of the sheet. My tank is essentially a trapezoidal-shaped box. Using a bit of modest trigonometry, I was able to work out the dimensions of all the sides. I used these dimensions to cut the bottom and the side pieces. However, after just the very first weld, inside piece to bottom, my dimensions started changing. I adjusted as necessary but the net result is that the tank wound up a bit smaller than planned. I think with enough clamps it might be possible to create a jig that would hold all the pieces (excepting the top) and tack weld them so that they would not flex during final welding, but I did not have the tools to do this.

With some re-cutting, I got the four sides and the bottom welded. For all of these welds I was able to put a bead of weld on the inside and outside, assuring a very tight and strong weld. The top would have to be welded-on using a single outside weld. However, top welds do not have the same loads as the side welds, so I I don’t think this is an issue. I also welded in a baffle that runs laterally across the tank to reduce sloshing and give the sides additional (and probably totally unnecessary) strength. I can tell you that 3/8” PP sheeting is really, really strong stuff.

Before welding the top on I considered all the plumbing connections the tank was going to require. There are five opening in the top:

- Bulkhead hitting for the input hose barb, 1.5”
- Two bulkhead fittings for the pump out barbs, 1.5”
- Vent, 0.5”
- Inspection port 4”

These holes were drilled with a drill press and standard round hole cutters of the appropriate size. The two bulkhead fittings for the drains are equipped with 1.5” PVC pipes that extend down to ¼” off the bottom of the tank. A test of this method demonstrated that priming of the macerator pump is nearly instant, due to the short lift height, and the pump will pick up all but about two quarts of the liquids in the tank. I also welded on a small square of PP sheet under the macerator mounting point to allow the use of longer fastening screws.

With all this completed, I welded the top panel to the tank. The fit was not as precise as I would have liked due to the warping, but plastic welding is forgiving and you can build up an area to bridge an opening. I tested the tank by putting two gallons of water in it and setting it on every side. The side and bottoms welds, which were double welded, were 100% tight. I did have four small leaks on the top welds, which were easily corrected.

Installation of the tank was simple. It had been designed to fit into the existing opening and space under the port side bunk. There is a cabinet door there where you can see the tank and inspect its level. After measuring the capacity of the tank, I half filled it and put a half-full mark on the side of the tank. I estimate that the tank will hold about six person days of waste.

As seen in the photo, all of the lines to the top of the tank. The macerator is located there with sufficient room to turn the shaft manually using a screwdriver, if necessary. Any hose removals will only result in a small, easily cleanable, amount of sewage escaping.

A test of the system has demonstrated that it works perfectly. The pump primes quickly and can drain the tank in less than a minute. I don’t know that this will solve the intermittent problems that I have had in the past, but now I will be able to see what is happening in the system and have easy access to all components should repairs be necessary.