5.5 Imhoff Tanks, Septic Tank Systems & Intermittent Sand Filters
Key Takeaways
- Imhoff tank systems are a named Group 3 process in 35 Ill. Adm. Code 380.115 and carry their own subject category on every Illinois wastewater exam, from one question on the Class 4 and Class 1 exams up to two on the Class 3 exam.
- An Imhoff tank stacks an upper flow-through settling compartment over a lower unheated digestion compartment, with sloped slots and a gas-venting scum chamber that keep rising digester gas out of the settling zone.
- Imhoff operation centers on three manual tasks: reversing flow direction weekly or twice weekly to even out solids, keeping the slot overlap clear with a chain or squeegee, and drawing digested sludge slowly from the bottom under hydrostatic head.
- Intermittent sand filters are a separate Illinois exam category worth three questions on the Class 4 exam; they dose 24 to 36 inches of clean medium sand at roughly 1 to 5 gpd/sq ft in batches, with rest periods that restore aerobic conditions between doses.
- Septic tanks and Imhoff tanks both rely on unheated ambient digestion and both must retain seed sludge at drawoff, which is why neither is ever pumped completely empty.
5.5 Imhoff Tanks, Septic Tank Systems & Intermittent Sand Filters
Illinois keeps three small-plant technologies alive as distinct exam subjects because hundreds of village and subdivision systems around the state still run them. Imhoff Tanks and Intermittent Sand Filters each appear as their own subject category on the Illinois EPA wastewater examination content list, and 35 Ill. Adm. Code 380.115 names Imhoff tank systems and sand filters under Group 3, alongside trickling filters and RBCs. Question counts published by the Agency make the weighting clear: Imhoff Tanks draw 1 question on the Class 4 exam, 2 on the Class 3 exam, and 1 each on Class 2 and Class 1; Intermittent Sand Filters draw 3 questions on the Class 4 exam and are concentrated there because lagoon-and-sand-filter villages are the Class 4 population.
1. The Imhoff Tank: One Structure, Two Stories
Karl Imhoff's 1906 design solves a problem that plagued early single-chamber settling tanks: sludge digesting on the floor releases gas bubbles that lift settled solids back into the clarified liquor. The Imhoff tank separates the two functions vertically inside a single concrete structure with no moving parts and no external power.
| Compartment | Location | Function | Typical design value |
|---|---|---|---|
| Settling (flowing) chamber | Upper, V-shaped trough | Plain sedimentation of raw sewage | 2 to 4 hours detention at design average flow |
| Sloping bottom & slot | Base of the V | One-way passage for settled solids | Sides sloped 1.25:1 to 1.4:1; slot 6 to 12 inches wide with a 6 to 12 inch overlap |
| Digestion chamber | Lower, hopper bottom | Unheated anaerobic digestion at ambient temperature | 4 to 12 months of storage, depending on winter temperature |
| Gas vents / scum chambers | Flanking the settling trough, typically 20% or more of the surface area | Release digester gas and hold digester scum away from the settling zone | Free vent to atmosphere |
Why the overlapping slot matters. Solids settling in the upper trough slide down the sloped walls and fall through the slot into the digester. The lip on the digester side of the slot overlaps the lip on the settling side, so gas bubbling up from digestion is deflected sideways into the gas vents instead of rising back through the slot. If the slot is allowed to plug with rags and grease, solids back up into the settling chamber, digester gas finds the path of least resistance through the slot, and the effluent turns black and septic almost immediately.
Routine Imhoff operation
- Reverse the flow direction weekly or twice weekly. Imhoff tanks are built with inlet and outlet gates at both ends. Solids accumulate preferentially under the inlet end. Reversing flow evens out the sludge profile and prevents a mound from building up to the slot.
- Clean the slot. Pull a chain, scraper or squeegee through the slot on the same schedule. This is the single most important manual task in Imhoff operation.
- Break and remove scum in the gas vents. Scum mats blanket the vents, trap gas and can float digester solids. Operators break the mat with a rake or water jet and skim accumulated grease. Adding lime to the vent can help control odor and pH.
- Draw sludge slowly under hydrostatic head. A drawoff pipe from the digester hopper runs to a drying bed or hauling truck. Sludge is pushed out by the liquid head above it, so the valve is cracked open and throttled. Never draw the digester down completely — a heel of digested sludge is the only seed inoculum the tank has, and losing it stalls digestion for months.
- Watch for "rising sludge" through the slot. Black, foaming liquor and gas breaking in the settling chamber means the slot is fouled, the digester is overloaded, or drawoff has fallen behind.
Imhoff tanks provide primary treatment only: expect roughly 40% to 60% TSS removal and 25% to 35% BOD removal, the same order as a conventional primary clarifier. Because they are unheated, Illinois winters slow digestion sharply, which is why storage volumes are sized in months rather than days.
2. Septic Tanks in Municipal Service
A septic tank is the Imhoff tank's single-chamber cousin: a buried, watertight, unheated vessel where settling, flotation and ambient anaerobic digestion all share one liquid volume. Illinois operators encounter them at small institutional systems, at facilities served by septic tank effluent pump (STEP) collection, and ahead of intermittent sand filters and subsurface disposal fields.
- Compartments and baffles. Two compartments (or two tanks in series) outperform one. Inlet and outlet baffles or sanitary tees extend below the liquid surface so flow enters and leaves from the clear middle zone rather than disturbing the scum mat or the sludge blanket. The outlet tee extends deeper than the inlet tee.
- Detention. Design detention is typically 24 to 48 hours at average daily flow.
- When to pump. The industry rule of thumb is to pump when the sludge blanket plus scum occupies roughly one third of the liquid depth, or when the bottom of the scum layer comes within about 3 inches of the outlet device, or the top of the sludge within about 12 inches of it. A sludge judge or a hinged "core sampler" measures both layers.
- Seed sludge again. As with the Imhoff digester, leave a residual layer when pumping.
- Effluent quality. Septic tank effluent is anaerobic, sulfide-bearing and typically 120 to 180 mg/L BOD — it is a pretreated, not a treated, wastewater, and it must go on to a filter, subsurface field or downstream plant.
3. Intermittent Sand Filters
An intermittent sand filter (ISF) is an aerobic biological and physical polishing process: settled effluent is dosed onto an open bed of sand in batches, percolates down through a biologically active surface mat, and drains to an underdrain. The bed then rests, drawing air down into the medium and restoring aerobic conditions before the next dose. That alternation between flooding and resting is what "intermittent" means, and it is the mechanism worth understanding for the exam.
Design and media specifications
| Parameter | Typical value |
|---|---|
| Medium depth | 24 to 36 inches of clean, washed sand |
| Effective size ($D_{10}$) | 0.3 to 0.6 mm |
| Uniformity coefficient | less than 3.5, preferably under 2.5 |
| Hydraulic loading (single pass, settled domestic sewage) | roughly 1 to 5 gpd/sq ft |
| Dosing frequency | 2 to 4 doses per day, distributed over the whole bed |
| Underdrain | Perforated pipe in washed gravel, vented to atmosphere |
| Typical performance | BOD and TSS commonly below 10 to 20 mg/L, with substantial nitrification |
Loading rate is everything. Because the biological mat forms at the sand surface, an ISF fails from the top down. Overloading — hydraulically or organically — keeps the surface saturated, drives the mat anaerobic, and the bed ponds. The corrective sequence is to take the bed out of service, let it dry and crack, then rake or scarify the top inch or two, and in a badly fouled bed remove and replace the top 1 to 2 inches of sand. Operators must dose alternating beds so that at least one is always resting.
Winter operation in Illinois. Beds are ridged or covered with loose straw or snow so that doses percolate beneath an insulating crust rather than sheeting across a frozen surface. Ice lensing at the surface is the cold-weather analogue of biological ponding.
Recirculating sand filters are the higher-rate variant: filtrate is returned to a recirculation tank at ratios around 3:1 to 5:1, allowing hydraulic loadings of roughly 3 to 5 gpd/sq ft of forward flow while improving denitrification and odor control.
4. Comparing the Three Processes
| Imhoff tank | Septic tank | Intermittent sand filter | |
|---|---|---|---|
| Biological regime | Anaerobic (lower chamber only) | Anaerobic | Aerobic |
| Illinois facility group | Group 3 (380.115) | Pretreatment ahead of another unit | Group 3 sand filters; ISF questions weighted to Class 4 |
| Energy / moving parts | None | None | Dosing siphon or pump only |
| Level of treatment | Primary | Pretreatment | Secondary / polishing |
| Key operator task | Slot cleaning and flow reversal | Measuring scum and sludge, timely pumping | Dosing rotation, scarifying, preventing ponding |
| Failure signature | Gas and black solids rising through the slot | Solids carryover past the outlet tee | Surface ponding and odor |
An operator at an Illinois Group 3 village plant finds black, gassy solids boiling up into the settling compartment of an Imhoff tank, and the effluent has turned septic. Which combination of causes and corrective actions fits this condition?
An intermittent sand filter serving a small Illinois community begins to pond, holding standing wastewater on the surface hours after each dose, and the bed smells septic. What has happened and what is the correct operational response?
Why must an operator leave a residual layer of digested sludge in an Imhoff tank digestion compartment or a septic tank when drawing solids?