The Imhoff Tank

Cut the acrylic to the setting-out from step one. Six plain panels for the box, two sloping hopper plates, two vertical baffles to form the gas vents, and a small inlet baffle. Cut acrylic slowly. It melts and re-welds behind a fast blade, and a rewelded kerf is a stressed edge that crazes later when solvent touches it. A fine-tooth blade at moderate speed, and let the offcut fall away freely. Bond with acrylic solvent cement applied by capillary action: hold the joint together, touch a needle applicator to the seam, and let the solvent run into it. Do not butter the surfaces. The solvent softens both faces and they fuse; excess solvent leaves a cloudy, weak joint. Work in a ventilated space. The order matters, because two of these joints are unreachable once the tank is closed. Build the base and the two long sides first. Fit the two sloping hopper plates next, checking the overlap dimension at the slot with calipers before the solvent sets — this is the one measurement that must be right, and it is the one you cannot correct afterwards. Fit the gas-vent baffles. Fit the ends last, and the second end last of all. Cut the inlet as a 20 mm hole in one end, 60 mm below the intended water line, with a short baffle inside it so the incoming flow is directed downward and its momentum is broken before it reaches the settling zone. An unbaffled inlet jet short-circuits the channel and the tank settles nothing, which is a failure mode worth recognising because it looks identical to a tank that is simply too small. Cut the outlet as a weir across the far end, a slot 20 mm deep with a hardboard or acrylic strip that can be adjusted for level. Fit a scum baffle 40 mm in front of it, dipping 50 mm below the water line, so floating material cannot leave over the weir. Finally, fit a 15 mm drain with a valve at the bottom of the digestion hopper. That is how sludge comes out, and a tank without one has to be tipped up to empty, which destroys three months of stratification in ten seconds.

Fill the tank with clean water first and check the weir is level and the slot dimension is what you set out. Run clean water through for an hour and dye-test it: inject a slug of food colouring at the inlet and watch. You should see it move as a broad front along the channel. If it streaks straight from inlet to outlet along one wall or dives to the floor, the inlet baffle is not breaking the momentum and must be enlarged before you put anything organic in. Seed the digestion chamber. Take two litres of sludge from an established anaerobic source — a septic tank, a farm digester, the bottom of a long-settled slurry pit — and introduce it into the lower storey through the sludge drain, not over the top. It must go in below the slot. This gives you the methanogenic organisms, which are slow-growing and which the tank will not otherwise acquire for months. Feed the same defined synthetic wastewater used for the trickling filter — per 10 litres of tap water, 1.5 g glucose, 1.5 g peptone, 0.5 g ammonium chloride, 0.5 g sodium bicarbonate — dosed at whatever flow the notebook told you gives a three hour detention. Never real sewage in a transparent indoor tank. Now the observation this whole build exists for. Within days to a couple of weeks, gas bubbles start rising from the digestion chamber. Watch where they go. They rise, meet the underside of the overlapping hopper plate, and run sideways along it until they reach the vent channel, where they break surface. Almost none come up through the slot. Put your eye level with the slot and watch for ten minutes: the traffic is downward, solids only. Compare this directly with a septic tank, where gas rises through the settling layer and continuously lifts sludge back into suspension. That comparison is the point of the invention and you can see it with no instrument at all. Two things to watch for as it matures. A scum blanket will form on the vents; that is normal, and it needs breaking with a rod weekly or it seals and drives gas back to the slot. And the sludge in the lower storey should stratify — actively digesting near the top, denser and more mineralised at the bottom. When you draw sludge, draw from the very bottom, slowly, and take only part of it: the resident sludge is your inoculum and emptying the chamber restarts the biology.