When sewage makes the news headlines, it's usually in the form of a 'pollution incident' - murky water pours from a pipe into a river, or beaches are declared unsafe for swimming. These events are striking because they show a sudden change in the natural environment, but do these dramatic happenings tell the entire story?

In rural Hampshire, many properties are not connected to the main sewer system, relying instead on private 'off-mains' cesspits, septic tanks, and small sewage treatments plants. In the right circumstances, these systems offer a solution to a complex problem, but when something goes wrong then disaster quickly looms.

The kinds of issues found in faulty or mismanaged off-mains systems cause something called 'chronic pollution'. In contrast to a pollution incident, this happens over a long period and isn't always immediately obvious. A under-performing system may slowly leak waste or release pollutants with its discharged wastewater.

As such, while the percentage of off-mains properties is small, they have the potential to massively damage their local areas. This is compounded by the fact that they often occur in clusters - sometimes entire villages. It might be slow, and it might be subtle, but if something is amiss then homes and nature are both at risk.

There are many factors that can affect the functioning of these systems, some of which are tricky to control. Like the mains network, they are somewhat at the mercy of their surroundings: a change in ground conditions, for instance, may add groundwater that 'squeezes' a system's tank until it cracks. The same result can be caused by tree roots, soil movement, or heavy vehicles.

In truth, though, many common problems stem from mismanagement. A 2015 Natural England report on septic tanks (the most common system) found that many are incorrectly sited, poorly maintained, or too small for their property. As a result, the concentration of phosphorus in nearby streams rose dramatically - up to 700% in some cases - with disastrous effects on wildlife.

Phosphorus, like nitrogen, is a highly effective fertiliser. Sewage is rich in both, and so encourages the rapid growth of algae. Algal 'blooms' hoover up the stream's dissolved oxygen and block out the sunlight. This kills the aquatic plants, which are then broken down by bacteria; a process that also uses oxygen. Eventually the algae themselves die, adding to the mass of rotting organic matter.

The result of this is a dramatic downturn in biodiversity. Many freshwater insects and fish need highly oxygenated water to thrive. If the level drops too low, their eggs fail to develop and the adults suffocate. These conditions also encourage the growth of cyanobacteria (or 'blue-green algae') - which can cause ill health in people, pets, and livestock - and various types of 'sewage fungus'.

In addition to these nutrients, human sewage often contains other harmful elements. Many of the things we eat and drink end up in our waste, and off-mains systems are incapable of removing them. Although their impact isn't fully understood, the existing research suggests that a variety of animals are negatively affected by substances like caffeine, ibuprofen, and oral contraceptives.

This danger goes beyond the streams themselves: the highly porous ground in our area makes it easy for pollutants to reach the chalk aquifer. This hidden store feeds the streams, but we also tap it for drinking water. The more polluted the source, the more treatment the water needs to be safe for consumption - this has both environmental and financial implications.

While the list of potential issues is daunting, you'll be pleased to hear that most of them are preventable. Using your drainage system in a responsible way will keep it in great shape, and a watchful eye will help you quickly tackle any faults that do occur. As a system user, some of this is required by law - check out our easy guide for keeping your system safe, legal, and effective.

We're helping off-mains users to keep their systems working at their best, through the Septic Smart project. If you're passionate about this pressing issue, why not get involved?

More about the Septic Smart project

How off-mains sewage systems work

How to manage your off-mains sewage system