What Is a Septic Tank?

Wastewater from the property enters the tank through the inlet pipe. Inside, the flow slows so materials can separate: ^[1]

• Heavier solids settle at the bottom and form sludge.
• Lighter material, including fats and grease, collects at the surface as scum.
• Natural biological activity breaks down some organic material, while solids continue to accumulate.
• The partly treated liquid, called effluent, leaves through the outlet for further treatment.

Baffles, dip pipes and chamber divisions help control flow and retain solids. Older brick-built tanks may use masonry dividing walls and clay or other pipe fittings. Identify the actual layout: damaged or missing parts can let solids reach the outlet and affect the receiving system. ^{[2, 3]}

A septic tank should contain its contents and direct effluent through the intended outlet. It is not designed to dispose of sewage through leaking walls or an open bottom. ^[3]

For a factory-made tank, look for its name, model and installation records. An older brick-built tank may have no manufacturer’s label or surviving drawings. A survey should establish its dimensions, chambers, connecting openings, inlet and outlet, and any later alterations.

Check how those components work together. Several covers do not necessarily mean several separate tanks, and a hose or cable alone does not identify the treatment process. The report should record the construction and layout found, with any concealed or inaccessible parts clearly identified.

The inspection needs to follow the wastewater from the incoming drains, through the tank and onward to its discharge. For a brick-built tank, the checks include: ^{[14, 15]}

• Brickwork and joints. Look for cracked, loose or displaced bricks, deteriorated mortar, roots, bulging walls and damage to any internal render. Check the visible base and roof slab where access permits.
• Chamber divisions and fittings. Check that dividing walls, connecting openings, baffles and dip pipes are intact and performing their intended job. A collapsed partition can allow sewage to bypass the intended settlement route.
• Liquid level and leakage. Compare the level with the outlet arrangement and recent use. An unexplained low level can indicate leakage; a high level or backflow can indicate restricted discharge. Look for groundwater entering through joints or pipe connections.
• Sludge and scum. Assess accumulation and whether the tank retains solids. Excess sludge reduces useful capacity, while solids escaping through the outlet can damage the receiving field.
• Connected drains and flow. Where appropriate, observe flow during controlled water use and inspect accessible foul drains for blockage, deposits or staining from previous backing up.
• Discharge route and receiving ground. Establish where the outlet leads. Check for a submerged outlet, ponding, surfacing effluent or other evidence that the receiving system cannot accept the discharge.
• Covers and access. Check lids, frames, supporting masonry and visible slab condition. Record damaged, loose or unsafe access points and access needed for maintenance.

These findings build the operational assessment. A toilet flushing freely or a tank having been emptied is not enough on its own. The report should state whether the evidence supports normal operation, identifies a defect, or requires further inspection or testing.

Liquid and sludge can conceal the lower walls and base. Record working levels first, then arrange controlled emptying if needed for inspection from outside. A visual inspection alone cannot prove every joint is watertight; suspected leakage may need a separate test. Recent emptying, low occupancy and dry conditions can also mask problems. ^{[14, 15]}

Sludge must be removed before it exceeds the tank’s maximum capacity. Environment Agency guidance recommends desludging at least annually or following the manufacturer’s instructions. Use a registered waste carrier. For a brick tank without instructions, ask a competent maintenance contractor about its care. ^[4]

Emptying removes accumulated solids; inspection checks condition and operation. With brick-built tanks, record masonry, chamber divisions, fittings and covers as well as flow. Keep maintenance records and arrange repairs to defects found. ^[8]

Agree safe tanker access and an emptying procedure suitable for the tank’s construction and groundwater conditions. Where an old brick tank has no instructions, the contractor should assess the arrangement before pumping it down, particularly in wet ground. ^[8]

Keep covers secure and arrange repairs to damaged lids promptly. Do not enter a tank or chamber: dangerous atmospheres and other confined-space hazards can be present. ^[12]

The recommendation should follow the findings. An older brick tank may be repairable where a local defect can be corrected safely and the remaining structure and arrangement are suitable. Extensive structural damage, an ineffective internal layout or an unsuitable discharge may require more substantial work. Establish whether repair can restore reliable operation before recommending replacement.

Before choosing septic tank or treatment plant replacement, establish where the effluent will go. Installing a treatment plant does not change the permeability of clay or create capacity in a saturated field. The discharge arrangement may need its own investigation, testing and design.