Home/Septic & Soakaway
Septic Tank and Soakaway Guide for London Properties
For London properties not connected to the main sewer, a septic tank and soakaway system is a common, regulated solution. Its long-term viability is not a matter of simple installation but of precise hydrological diagnosis and ongoing documented maintenance, as failure poses significant environmental and legal risks.
Quick answer
A septic tank is a primary settlement chamber for sewage, followed by a soakaway which provides tertiary treatment and dispersal. In London, their design and function are critically dependent on a detailed percolation test and CCTV survey of the drainage field.
Our approach is forensic: we treat every system as a unique hydraulic puzzle defined by London's challenging geology. This guide explains the technical principles, the mandatory diagnostic procedures, and the evidence-based maintenance required to prevent system failure and regulatory non-compliance.
The Hydraulic Function: Tank Settlement and Soil Percolation
A septic tank is a watertight, multi-chambered vessel designed for primary treatment. Inflowing wastewater undergoes settlement, where solids form sludge, and partial anaerobic digestion. The clarified effluent then discharges to the soakaway system. Crucially, the tank only reduces solids and biological oxygen demand (BOD); it does not 'treat' the effluent to a safe standard.
The soakaway, or drainage field, is the critical secondary component. It consists of perforated pipes laid in gravel-filled trenches within unsaturated soil. Treatment occurs via aerobic bacterial digestion within the soil matrix (the biomat) and filtration. The system's entire capacity is determined by the soil's percolation value (Vp), measured in seconds per millimetre, which dictates the required drainage field area.
Mandatory Diagnostic: The Percolation Test and CCTV Survey
Compliant design under British Standard BS 6297:2007 begins with a percolation test. This is not a visual assessment but a quantified measurement of the soil's absorption rate. We excavate test pits to the proposed invert level of the drainage field, saturate the soil, and then time the drop in water level. This yields the Vp value, from which the precise area of drainage trench is calculated.
Prior to any design or repair, a full-system CCTV survey is non-negotiable. For existing soakaways, we utilise a crawler camera to traverse the perforated pipes, documenting the internal condition. We look for evidence of hydraulic failure: root intrusion, sediment smothering the biomat, structural collapse, or permanent saturation indicating a rising water table or inadequate percolation.
- Test Pit Excavation: To proposed drainage field depth and dimensions.
- Soil Saturation & Measurement: Standardised soaking and timing procedure.
- Vp Calculation: Determining seconds per mm drop for design compliance.
- CCTV Crawler Deployment: Visual documentation of the entire drainage field lumen.
- Biomat Assessment: Analysing camera footage for signs of clogging or failure.
London-Specific Failure Modes and Diagnostic Evidence
London's prevalent clay subsoil and high groundwater tables are the principal adversaries of soakaway function. Clay has a very slow percolation rate, often requiring an impractically large drainage field. A high water table removes the unsaturated zone necessary for aerobic treatment, causing the system to hydraulically lock and discharge untreated effluent.
Diagnostic evidence of failure is gathered methodically. We correlate CCTV findings with drain jetting patterns; if water returns immediately or fails to dissipate, it indicates a saturated field. Dye testing can confirm illicit discharges or overflows. The culmination is a detailed report with timestamped video evidence, still images with annotations, and a clear diagnosis of the failure mechanism.
- Clay Soil: Low Vp value leading to surface breakout or backflow.
- High Water Table: Saturation observed during CCTV survey and testing.
- Biomat Clogging: Evidenced by slow drainage and sediment on camera.
- Structural Failure: Pipe collapse or deformation visible via CCTV.
Documented Compliance: Building Regulations and Environmental Permits
Installation or significant repair of a septic tank serving a domestic property must comply with Building Regulations Part H2. Our documentation for compliance includes the percolation test report, system design calculations, pipe layout plans, and evidence of installation to spec. For systems discharging more than 2 cubic metres per day to ground, or any to a watercourse, an Environment Agency Environmental Permit is required.
Since January 2020, General Binding Rules prohibit the discharge of septic tank effluent directly to a watercourse. Any such existing system must be upgraded to a full sewage treatment plant or a compliant drainage field. We provide the diagnostic evidence needed to prove compliance or justify upgrade works, protecting you from enforcement action.
Procedural Maintenance: Jet Vacuuming and Scheduled CCTV Audits
Maintenance is a scheduled, documented procedure, not a reactive task. The septic tank requires regular desludging by a licensed carrier (typically every 12 months) to prevent solids carry-over into the drainage field, which clogs the biomat. We employ jet vacuuming units that first jet the drainage field pipes to break down superficial blockages, then immediately vacuum out the dislodged silt, documented via camera before and after.
We recommend a biennial CCTV audit of the soakaway. This establishes a baseline and tracks the progression of any issues like root growth or sediment accumulation. This proactive, evidence-led approach allows for planned intervention, such as localized liner repair or targeted root removal, avoiding catastrophic and costly total failure.
Fixed price before we travel
Every visit is priced as a clear fixed fee — not an open-ended hourly meter that climbs while someone guesses.
CCTV-confirmed, not assumed
A camera check confirms the drain is genuinely clear or the defect is genuinely diagnosed, not just assumed from flow.
Documentation that holds up
Footage, defect codes and a written report when a managing agent, surveyor or insurer needs more than a one-line invoice.
Our services
Related drainage services
London coverage
Available across all 33 boroughs
Same approach from Barking to Richmond. Browse a borough for local housing notes, or send your postcode for a fixed-price slot.
Frequently asked questions
How do I know if my septic tank soakaway has failed?
Diagnostic signs include persistent foul odours, soggy ground or lush vegetation over the drainage field, and slow-draining or backing-up toilets. A definitive diagnosis, however, requires a CCTV survey. Our engineers will document the internal condition of the drainage pipes, looking for standing effluent, root intrusion, or sediment, providing video evidence of the failure mode.
What is a percolation test and why is it critical in London?
A percolation test is a standardised procedure to measure the soil's rate of water absorption (Vp value). It is the fundamental data point for designing a compliant drainage field. In London, with its dense clay and variable water tables, this test is critical. Without it, a soakaway is likely undersized and will fail, as the design is based on guesswork rather than hydraulic calculation.
Can a failed soakaway be repaired, or does it need replacing?
This depends entirely on the diagnostic evidence. Localised issues like a root-blocked section or a collapsed pipe may be repaired via no-dig lining or excavation. However, systemic failure due to inadequate soil percolation or a high water table often necessitates a complete redesign. This may involve a new drainage field in a suitable location, or conversion to a sewage treatment plant with a different discharge route.
What documentation should I have for my septic tank system?
You should hold the original design calculations and percolation test report, Building Regulations compliance certificate, records of all desludging by licensed carriers, and reports from any CCTV surveys or maintenance works. This documented history is vital for proving compliance, property sales, and planning effective repairs. We provide detailed digital reports for all diagnostic and maintenance visits.
My system discharges to a ditch. Is this still legal?
No. Under the General Binding Rules introduced in 2020, the direct discharge of septic tank effluent to a surface water body is prohibited. This system is now non-compliant. You must upgrade to a full sewage treatment plant (which produces a cleaner effluent suitable for watercourse discharge) or install a compliant soakaway. We can conduct a full diagnostic survey to determine the most viable upgrade path for your property.
Keep reading
Related guides
Technical Guides · 4 min read
Identifying the Signs of a Blocked Drain at Home: A Diagnostic Guide
Recognising the early signs of a blocked drain is crucial for preventing major issues. This guide explains the diagnostic indicators and the role of a CCTV survey in providing documented evidence of the blockage.
ReadTechnical Guides · 4 min read
The Real Cost of Cheap Drain Unblocking in London: A Technical Assessment
A cheap, one-off drain unblocking often masks deeper problems, leading to repeat callouts and higher long-term costs. We explain why a diagnostic CCTV survey is the only cost-effective starting point.
ReadGot a drainage problem in your London home?
Tell us the symptoms and your postcode. Fixed price, agreed arrival window, confirmed before you book.
Book a drainage visit