Why this matters for the Dart: in a rural catchment, hundreds of homes are their own tiny sewage works, and a neglected one quietly pollutes its stream every day.
The rule: if your septic tank discharges directly into a stream, river or ditch, it does not meet the law and must be replaced or upgraded. And every system, however good, needs emptying regularly.
If your home is not on mains drainage, your wastewater is treated on your own land, usually by one of three systems. A septic tank is a buried settlement tank that partially treats sewage and releases the liquid to a drainage field (a network of buried perforated pipes that lets it soak safely through the soil). A small sewage treatment plant (or "package treatment plant") uses a powered process to treat sewage to a much higher standard. A cesspool is simply a sealed holding tank with no outlet, which must be emptied whenever full 54. It is worth finding your paperwork, or your tank lid, and knowing which you own.
In England, small private systems are governed by the Environment Agency's general binding rules 48 49. These are the essentials.
Septic tanks must not discharge to a watercourse. Under the rules, a discharge to a river, stream or ditch must come from a small sewage treatment plant, not a septic tank. If your septic tank discharges directly to a watercourse, you must act as soon as possible: connect to the public foul sewer, install a drainage field so the discharge goes to ground instead, or replace the tank with a treatment plant. The Environment Agency expects plans to be in place within a reasonable timescale, usually 12 months 48. This requirement is often called "the 2020 rule", because 1 January 2020 was the original compliance deadline set when the rules changed; the current guidance refers to sales made before that date 48. Why it helps the Dart: a septic tank's liquid outflow is only partially treated, and piping it straight into a small stream is a permanent pollution source. How hard: a bigger project, and a significant cost, but a legal requirement, and it removes your home from the river's problem list for decades.
Selling the house brings it to a head. If you buy or sell a property whose septic tank discharges to a watercourse, the guidance says buyer and seller should agree who will replace or upgrade the system, as a condition of the sale. Every seller of a property with any private sewage system must also tell the new owner in writing that a discharge is in place, with a description of the system and its maintenance records 48 49. How hard: paperwork, but essential.
Stay within the size limits. The rules allow a maximum of 2 cubic metres (2,000 litres) a day discharged to ground, or 5 cubic metres (5,000 litres) a day to a watercourse from a treatment plant; beyond that, or in certain protected locations, you need a permit 48 49. For an ordinary house these limits are generous, but they matter for shared systems and holiday accommodation.
Empty (desludge) the tank regularly. You must have sludge removed before the tank exceeds its maximum capacity, and the guidance is to desludge at least once a year or in line with the manufacturer's instructions, using a registered waste carrier 48 49. Why it helps the Dart: a full tank pushes solids out into the drainage field or watercourse, and a clogged drainage field cannot be repaired cheaply. How hard: easy, one phone call a year; expect a modest cost and keep the receipts with the house documents.
Buy the right kit if you replace anything. New or replacement equipment must meet the relevant British Standards, BS EN 12566 for tanks and small sewage treatment plants and BS 6297:2007 for drainage fields 48 49.
Run a "See and Smell" check every few months. Catchment charities running septic tank campaigns suggest simple self-checks between emptyings 51 50: drains slow to clear or toilets backing up; spongy ground or unusually lush grass and vegetation over the drainage field; and dark, smelly liquid, or anything containing solids, at the discharge pipe 51. Any of these suggests the system is failing. Why it helps the Dart: failing systems announce themselves early to anyone who looks. How hard: easy.
Protect the bacteria that do the work. A septic system is a living thing: bacteria digest the sewage. Do not overwhelm them with bleach, antibacterial cleaners, disinfectants or chemicals, choose phosphate-free products, and never pour fats, oils, medicines, paints or solvents down the drain 25 54. Keep rainwater out of the system too; gutters and drives should never drain into it 54. Why it helps the Dart: a healthy tank treats sewage properly; a poisoned one passes it through. How hard: easy.
Elsewhere in the country, funded septic upgrade schemes have been run by councils, rivers trusts and partnerships, from Wiltshire's fully funded Revamp Your Tank grants to the Our Rivers free-upgrade partnership in Norfolk 52 53.