Size the digester, see what it needs330-265-1776 · Sales@TanksandCovers.com
Interactive Digester Sizing Tool

Size a bolted steel digester, and see everything it drags along with it.

Diameter and height are the two numbers everyone starts with. They are also the two numbers that quietly set the boiler, the insulation package, the mixer count, the gas storage buffer, and the plate thickness at the bottom ring. Drag them here and watch all of it move together on a live 3D tank — before any of it is committed to a drawing.

Open the Designer Get a Real Budget Number

What the tool returns

  • Volume & flowWorking volume, gross shell volume, and feed rate at your retention time
  • BiogasVolatile solids loaded, gas production, and the energy in it, on your feedstock's own conversion basis
  • HeatFeedstock heating, envelope loss, peak demand, and boiler size with margin
  • InsulationAn R-value target for your design low, and the heat it takes off the boiler
  • MixingInstalled kilowatts, mixer count, and motor size for side-entry units
  • Gas & structureDesign pressure, relief settings, storage buffer, and bottom-ring plate
  • Shell materialStainless rings where the H₂S attacks, epoxy below, and what the mix costs
Live model

Size it in real time.

Drag the diameter and height, or add a ring and watch the tank grow. Every downstream requirement — heat, insulation, mixing, gas pressure, flow, and shell material — updates with it.

—working volume
Figure shown is 6 ft tall
Drag to orbit · scroll to zoom
Why bolted steel

A digester asks for four things at once. Bolted steel is where they meet.

Digesters are corrosive, gas-tight, heated vessels that owners want built fast and often want to expand later. Those four demands are exactly where factory-coated bolted steel separates from cast-in-place concrete and field-welded steel.

Epoxy cured in a factory — or stainless, with no coating at all

Factory-applied thermoset epoxy is baked and holiday-tested on every panel before it ships. For the harshest headspace service — hydrogen sulfide, sulfuric acid at the gas–liquid line, biogenic corrosion — 304 or 316 stainless panels take the coating question off the table entirely, and the two can be mixed ring by ring.

No hot work, no cure time

Bolted erection is mechanical. There is no welding or grinding on site, no coating touch-up over weld seams, and no 28-day concrete cure in the schedule. A tank of the size above is typically erected in weeks rather than months.

Expandable when the feedstock grows

Panels unbolt. A bolted digester can be raised with an additional ring or relocated when a project is phased. Concrete and welded shells are permanent decisions made on day one, with day-one information.

Lighter foundation load

A steel shell weighs a fraction of a concrete wall of the same height, so the ring wall or slab carries mostly the liquid — lower foundation cost and fewer surprises on soft sites. See foundation systems.

Insulation and heating integrate cleanly

Rigid insulation and a metal weather jacket attach to the shell stiffeners. Internal or external heat exchangers, nozzles, manways, and side-entry mixer nozzles are factory-cut and reinforced — not core-drilled after the fact.

Gas-tight by design

Gasketed lap joints and sealed hardware make the shell suitable for low-pressure gas service. Bolted cone roofs and double-membrane gas holders both mount directly to the top ring.

Digester tank construction methods compared
DecisionBolted steel (factory coated)Cast-in-place concreteField-welded steel
Erection timeWeeks; runs parallel with foundation cureMonths; forms, pours, cureWeeks to months; weather-dependent
Corrosion protectionFactory-baked epoxy, holiday-tested per panel, or bare stainlessLiner or coating applied on site; crack-proneField blast and coat over weld seams
Headspace H₂S attackStainless upper rings and roof, epoxy belowBiogenic sulfuric acid etches concreteDepends on field coating quality
Expansion / relocationAdd rings, unbolt, moveNot practicalCut and re-weld; recoat
Site hazardsNo hot work on a gas vessel siteHeavy forming, rebar, pumpingWelding and grinding on a gas vessel site
FoundationRing wall or slab; light shell loadHeavy; wall self-weight is largeRing wall or slab

Comparison reflects typical North American practice for anaerobic digestion service. Project conditions — seismic category, freight distance, local labor, and gas system scope — can change the ranking, which is what a lifecycle cost comparison is for.

Methodology

What the designer is actually doing.

Every number on the panel comes from a handful of relationships that govern anaerobic digestion. Change one input and you can see which requirements move with it — and which ones do not.

Retention time sets flowFeed rate is working volume divided by hydraulic retention time: Q = V ÷ HRT. A bigger tank at the same HRT processes more feedstock; a longer HRT on the same tank means less throughput but more complete conversion.
Heat is mostly the feedstockWarming incoming material from its delivered temperature to 98 °F — or 131 °F thermophilic — usually dominates, at roughly 8.34 Btu per gallon per °F. Envelope losses through wall, roof, and floor are added at the design low ambient using the R-value you set.
Insulation is a climate decisionThe recommendation steps up as the design low drops. The savings line shows how much boiler capacity the insulation removes from the heating bill — and the first ten R-values do most of the work.
Mixing scales with volumeInstalled mixing power is set as watts per cubic meter of working volume, then split across a mixer count that keeps each unit in a practical motor range for side-entry propeller mixers mounted through the bottom ring.
The feedstock sets the gasEach feedstock carries its own volatile-solids destruction, gas yield per pound destroyed, and methane fraction. Choosing one loads a typical loading rate and delivered temperature; nudging the loading rate by hand switches the basis to custom and leaves the rest alone.
Stainless goes where the attack isPick how many rings from the top are stainless. Biogenic sulfuric acid forms in the headspace and at the liquid line, so the top rings and the roof take the most abuse — epoxy below the liquid line keeps the shell cost in check.
Gas pressure is low, but it is a pressureBolted digesters run inches of water column, not psi. Relief and vacuum settings, gas storage volume, and buffer hours all follow from the headspace and roof type you choose.
Hydrostatic load sets the plateWater pressure at the floor is 0.433 psi per foot of depth. Bottom-ring plate thickness is estimated from hoop stress with a corrosion allowance — the manufacturer's stamped design is final.

What the model deliberately excludes

Seismic and wind design, snow load on the roof, foundation bearing capacity, freeboard for foam, gas train and flare sizing, digester gas cleanup, and anything about the site's electrical service. It also assumes a single continuously stirred vessel: staged digesters, external heat recovery loops, and thermal hydrolysis pre-treatment all change the heat balance materially.

Biogas yield is taken as a typical volatile-solids conversion and methane fraction for the feedstock you pick. Real yield depends on feedstock, co-digestion recipe, and how completely the tank is mixed — a number worth measuring on your actual substrate before it becomes a revenue assumption.

Where to take it next

Once the geometry is settled, the bolted tank configurator turns it into a budget proposal, a preliminary general arrangement drawing, and an editable CSI three-part specification. Insulation scope is detailed on tank insulation systems, the heating loop on heat transfer equipment, and the mixers and chopper pumps under metering, pumping & mixing.

Questions

What engineers ask when sizing a digester.

How big does an anaerobic digester tank need to be?
Working volume is set by the flow you have to treat and the hydraulic retention time the process needs, not by a preferred tank size: volume equals daily feed rate multiplied by retention time. A mesophilic digester on municipal sludge is commonly designed for 15 to 25 days, and high-solids or co-digestion work often runs longer. Size the volume first, then choose the diameter and height combination that delivers it — the designer above shows how depth-to-diameter ratio, mixing arrangement, and heat loss all shift as you trade one dimension against the other.
Why use a bolted steel tank for a digester instead of concrete?
Four reasons that matter on a digester specifically. The corrosion protection is applied and verified in a factory rather than on a jobsite, and the upper rings and roof can be stainless where hydrogen sulfide and biogenic sulfuric acid attack the headspace. Erection is mechanical, so there is no hot work on a gas vessel site and no 28-day concrete cure in the schedule. Panels unbolt, so the tank can be raised with another ring or relocated when a project is phased. And a steel shell weighs a fraction of a concrete wall, so the foundation carries mostly liquid load.
How much heat does a digester need?
Usually the dominant term is raising incoming feedstock from its delivered temperature to the operating temperature, not envelope loss. Heating water-like feedstock takes roughly 8.34 Btu per gallon per degree Fahrenheit, so a large flow with a cold winter feed can easily exceed the loss through an insulated wall and roof. Envelope losses are then added at the design low ambient using the R-value of the insulation system. The tool separates the two so you can see which one your insulation and boiler decisions actually move.
How much insulation does a digester tank need?
It is a climate decision rather than a fixed number. In mild climates R-13 is often adequate; below roughly 20 °F design low, R-19 to R-25 on the shell and roof is a more defensible target. The economic return is front-loaded: the first ten R-values remove most of the loss and the curve flattens after that, which is why the calculator reports both the uninsulated envelope loss and the loss at your chosen R-value. Rigid board with a metal weather jacket attaches to the shell stiffeners on a bolted tank.
How much mixing power does an anaerobic digester need?
Mechanical mixing in a continuously stirred digester is typically installed at about 3 to 10 watts per cubic meter of working volume, with the higher end reserved for thicker feedstocks and tanks whose geometry works against circulation. Installed power is then divided across a mixer count that keeps each motor in a practical range for side-entry propeller units mounted through the bottom ring. Depth-to-diameter ratio matters: a tall narrow tank generally needs a second tier or a draft tube to keep the upper column turning.
What gas pressure does a bolted digester roof handle?
Digesters operate in inches of water column, not psi. A bolted steel cone roof for low-pressure gas service is commonly designed around 12 inches of water column — roughly 0.43 psi — while a double-membrane gas holder typically operates near 8 inches. Both need pressure and vacuum relief sized for the full gas production rate plus the liquid displacement that occurs during filling and draw-off, because the vacuum case during pumping down is the one most often undersized.
Can I use these numbers in a design submission?
Treat them as planning-level screening figures for scoping a tank and comparing arrangements, not as a stamped design. The relationships used are standard anaerobic digestion and heat transfer arithmetic with typical coefficients, and they are accurate enough to size a budget, a boiler range, and a mixer count. Final working volume, plate schedule, seismic and wind design, foundation, and gas system sizing are confirmed by the tank manufacturer's engineering and the project engineer of record for the specific site and code.

Send us the numbers. We'll turn them into a tank quote.

Copy the design summary from the panel above and send it over. We come back with a bolted steel digester proposal — epoxy-coated or stainless shell, cone roof or membrane gas holder, insulation, heating, mixing, and accessories — sourced and coordinated through our manufacturing partners, nationwide.

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Already operating a digester and weighing a repair against a replacement? An inspection and condition assessment establishes what the existing shell has left, and why tanks fail covers the headspace corrosion modes that end most of them.