Receiving Tankered Cesspool and Septic Waste at a Sewage Works: Strength, Inhibition, Reception Design and Load Limits

What arrives in the tanker

Tankered domestic waste in the UK is classified under European Waste Catalogue codes 20 03 04 (septic tank sludge), 16 10 02 (cesspool and portable toilet waste) and 20 03 06 (waste from sewer cleaning), and most water companies accept it at designated works under a haulier agreement with per-load and per-cubic-metre charges. Composition varies enormously because it depends on how long the tank stood, whether it was a sealed cesspool (all liquid retained, typically 18 m³ minimum under Approved Document H) or a septic tank (liquid overflows to a drainage field, sludge accumulates for years), and whether portable toilet additives are present.

Design values are those adopted in the Ontario guidelines from the US EPA septage survey, whose raw range is wider still (BOD₅ 440 to 78,600 mg/L, total solids 1,100 to 130,000 mg/L). Faecal sludge surveys give COD to BOD ratios of 6 to 12 against about 2 for sewage: a large part of the organic matter has already been through months of anaerobic digestion and is slowly biodegradable, so BOD understates the oxygen demand and the sludge yield at the works. Cesspool liquor from frequently emptied tanks is weaker and fresher, as the Polish figures show, and is better described as concentrated sewage than as sludge.

Effects on the biological process

Organic and solids shock. A two-year study at a small Polish works receiving tankered cesspool waste at a median 5.7 percent of inflow volume (range 2.1 to 12.4 percent) found the unit BOD₅ load on the biological stage rose to a median 90.5 g per person per day against a design value of 60, a 51 percent overload, with COD 29 percent and total nitrogen 42 percent above design. Regression on the same data gave unit BOD load (g/PE/day) of about 33.6 + 11.2 × (volume share in percent), so each percent of imports by volume added roughly 18 percent to the design organic load. Volume share is therefore a poor control variable; load share is the right one.

Respiration inhibition. Portable toilet fluids are the main chemical hazard. Formaldehyde-based products (20 to 30 percent formaldehyde) and bronopol-based products (1 to 30 percent) reduced activated sludge respiratory activity by 37 to 95 percent in OECD respirometry at the concentrations recommended by the manufacturers, with sludge from a works already receiving mobile toilet waste showing partial acclimation. Citric acid and limonene-based fluids were not inhibitory and were used as substrate. Nitrifiers are the most sensitive population, and because nitrification recovers over days to weeks, a single inhibitory load can breach a daily ammonia limit for longer than the load itself lasts. Event and festival waste should be screened for product type at the acceptance stage, and formaldehyde products refused or restricted at small works.

Septicity. Tankered waste is fully anaerobic, with redox potential below −200 mV, dissolved sulphide typically 5 to 50 mg/L and high volatile fatty acids. On discharge to an open channel the hydrogen sulphide strips immediately, giving odour complaints and corroding concrete and steel; in the aeration tank each kilogram of sulphide consumes about 2 kg of oxygen and sulphide above a few mg/L inhibits nitrification. Volatile fatty acids are useful carbon for denitrification or biological phosphorus removal if dosed into the anoxic zone, but when fed at the inlet of an under-loaded plant they favour low F/M filamentous bulking. Control measures are those for any septic input, covered in the sewer septicity guide: contained reception, extraction to an odour filter, ferric or nitrate dosing in the buffer tank, and discharge into a well-aerated point rather than a quiescent channel.

Grease and solids. At 8,000 mg/L grease and 15,000 mg/L solids, one tanker can deposit more fat on the inlet works than a day of sewage. Where imports are discharged straight to the inlet channel the usual symptoms are blinded fine screens, overloaded grit removal and scum on primary tanks.

Reception facility design

A reception facility should provide, in flow order:

• Access and identification. A hard-standing bay with drainage back to the facility, out-of-hours access by key fob or card linked to the haulier agreement, a weighbridge or a magnetic flow meter on the discharge line, and automatic logging of haulier, source, volume and time. UK undertakers increasingly use unattended logging stations for this.
• Connection. A 100 mm quick-release coupling in a sealed cabinet with a non-return valve, drip tray and wash-down point, so the tanker discharges under its own pump into a closed pipe rather than into an open channel.
• Sampling and rejection. An automatic sampler on the discharge line and online pH, conductivity and temperature with alarm limits (pH outside 5.5 to 9, conductivity above about 5 mS/cm) that close the inlet valve. Load refusal must be possible at the coupling, before the waste has entered the buffer.
• Screening and grit. A rock trap followed by a 6 mm screen with integral washing and compaction, sized for the tanker pump rate (typically 30 to 60 m³/h rather than the daily volume), then a grit channel or vortex unit. Screenings from septage are far higher per cubic metre than from sewage and the washer must handle rags, wipes and grit together.
• Buffer storage. A covered, mixed tank sized for at least one to two days of maximum imports, with level control, odour extraction to a carbon or biofilter and provision for ferric or nitrate dosing. Mixing homogenises the load across tankers of very different strength and keeps solids in suspension for metered release.
• Metered bleed. A variable-speed pump or control valve that releases the buffer to the process at a set rate over 20 to 24 hours, preferably during the overnight flow trough, with the rate interlocked to inlet flow and, where available, aeration dissolved oxygen.

Odour from the bay, cabinet and tank headspace falls under the works environmental permit and the Environment Agency odour guidance; enclosure with extraction at 5 to 10 air changes per hour to a treatment unit is the normal expectation at a works with neighbours.

Where to dose the waste

Three destinations are used. Dosing to the head of the liquid train, after the works own screens, is the simplest and suits cesspool liquor and dilute septage; the whole load passes through primary settlement, where most of the solids and grease drop out, and the biological stage sees the soluble fraction. Dosing to the primary sludge or digester feed suits thick septic tank sludge: the solids go where they belong, the digester gains gas from sludge that is still partly digestible, and the liquid train sees only the digester liquor return, but the digester must have capacity and the sludge must be free of chemical toilet biocides. Dosing to the anoxic zone as a carbon source exploits the volatile fatty acids but demands a buffer that has been settled or screened to a few hundred mg/L of solids. Works without primary tanks, including most package and small activated sludge plants, have only the first option and should set tighter limits, since nothing stands between the tanker and the aeration basin.

Setting the acceptance limit

A load rule should be checked determinand by determinand against the spare capacity of the works, not expressed as a volume. A commonly applied starting point is that imports should add no more than 10 percent to the daily BOD load the works was designed for, with solids, grease and ammonia checked separately; the Ontario guidelines frame the same test as the unused capacity above the current sewered load, which is the more defensible form where a works is already near its design load.

Bleed rate follows from the limit: 8.6 m³ released over 20 hours is 0.43 m³/h, or 7 L/min, against a DWF of 83 m³/h, so the instantaneous dilution is roughly 200 to 1 and the concentration step seen by the process is a few percent. Released as a slug over one hour the same tanker would be 10 percent of instantaneous flow and would depress aeration tank dissolved oxygen for the rest of the shift. Where the works nitrifies, the oxygen demand should also be checked: 60 kg BOD plus the slowly degradable COD behind it is 90 to 120 kg of additional oxygen per day, which a plant running near its blower capacity in summer may not have.

For small works the British Water code of practice for package plants is explicit that chemical toilet waste must not enter the treatment unit at all and should go to a separate cesspool for off-site disposal; the same logic applies to any works below about 2,000 PE, where one tanker exceeds any reasonable load rule.

Commercial and regulatory framing

Hauliers must be registered waste carriers and the receiving works must be permitted to accept the waste; acceptance conditions, sampling and rejection criteria form part of the haulier agreement. Charging in 2026/27 ranges from flat rates of around £15 per cubic metre plus an administration fee per load for domestic septic and cesspool waste to strength-banded tariffs for trade wastes rising above £50 per cubic metre for the strongest bands, so the facility should be metered and sampled well enough to apply the tariff. On the engineering side, the real cost of an import is the capacity it consumes: using the figures above, one tanker a day occupies about a tenth of the BOD capacity of a 10,000 PE works, equivalent to the headroom for 1,000 new residents, which should be visible in the growth planning for the works and in any discussion with the regulator about load-based consents. Inlet works and screening equipment are described in the equipment pillar, and the design of the downstream activated sludge stage in the activated sludge guide.