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Septic Systems and the Chalk Stream
a cess pit system, which is a large enclosed tank which has to be emptied when full

Image: H & IOW Wildlife Trust

Common Aspects

The village is completely off-mains for its sewage waste, each of us owning or sharing a septic system. This waste undergoes a natural separation of solids and liquids due to gravity, and is increased by bacteria performing both aerobic (oxygen-based) and anaerobic (non-oxygen based) digestion. The bacteria use up some of the waste as they grow and reproduce.

These septic systems are healthiest when teaming with bacteria and in a healthy tank the draining water will be fairly free of solid contaminants but will still contain chemicals from our domestic waste which have a negative effect on aquatic life.

entrance to a septic system

Image: H & IOW Wildlife Trust

standard septic system with the two settlement tanks and an outflow into a gravel soakaway

Image: H & IOW Wildlife Trust

Types of Septic Systems

A closed system (far above) is commonly called a Cesspit. This is simply a large tank, which when full, needs to be emptied. Nothing discharges into the ground.

A Septic System (above) is made up of two tanks and is the most common type. The first tank settles the majority of the solids, and the water then flows into a second tank to repeat the process before finally discharging out into a gravel soakaway where further filtering and bacterial work is done. Sometimes, the two tanks are separated from each other, with an electric pump from the first to the second - this is called a Micro Pump Station.

More recently, Package Plants are installed (right) - there is a third tank in between the two where air is introduced via a motor, increasing bacterial breakdown to the point where a gravel soakaway is not needed and the water discharges straight into the ground.

package plants add an aeration process into the septic system to make it efficient, removing the need for a gravel soakaway

Image: H & IOW Wildlife Trust

some of the key issues putting pressure on the chalk stream

Image: Mark Allen

Why Are We Worried?

Our chalk stream, the Arle itself and further downstream the Itchen waterways are all showing signs of environmental stress caused by chemicals and excessive nutrients, some of which are legacy from decades ago but are now coming out of the ground and also are accumulating from the waste water coming from our own septic tanks.

Water flowing from healthy septic systems contain chemicals and nutrients which can be harmful to aquatic life - small doses is ok, however excessive or faulty systems increase that significantly

This is exacerbated by the factors (above), all of which contribute to the pressures on the natural system.

from the bridge at Bighton Lane, 2016

Image: Mark Allen

sewage bloom

Image: H & IOW Wildlife Trust

Sewage Pollution

When tanks are not working properly, they pollute the ground, which eventually ends up in the streams and rivers. Also, many of our day to day chemicals, including medicines, microplastics and cleaning products are known to have a detrimental effect on the flora and fauna.

Sewage pollution is usually chronic and/or accumulative (individual systems each contributing a small amount, times the 160-ish houses in the village), which is harder to spot. This will be exacerbated by low water flows in dry weather or in very wet weather when abstraction levels are high.

This increases the cost of drinking water production, harms wildlife and changes the ecosystem and interferes with our enjoyment.

algal bloom caused by sewage

Image: H & IOW Wildlife Trust

algal blooms can choke the river

Image: H & IOW Wildlife Trust

Algal Blooms

Algal blooms are the direct result of sewage pollution.

Sewage is rich in nitrates and phosphates which while being natural nutrients, increases many-fold the existing nutrient levels and upsets the delicate balance of the ecosystem. Algae take up these nutrients very quickly and bloom exponentially in range. This in turn blocks out essential sunlight, uses up oxygen and smothers the habitat for other plant and aquatic life.

algal blooms can choke the river

Image: H & IOW Wildlife Trust

phosphates increase the nutrients for certain flora, which then strangles the stream

Image: H & IOW Wildlife Trust

Phosphate Monitoring

Phosphate monitoring measures the bioavailable form of phosphorous which is naturally present in small quantities. There is legacy phosphate in the ground and groundwater from past use (but no longer used) of phosphate-rich fertilisers and cleaning products.

Phosphate is the most common cause for English rivers to fail their ecological health statuses. Most phosphates cannot be removed from off-mains septic systems. These are a small contributor nationally but can have a big localised impact, especially in headwater areas like our village.

Image: H & IOW Wildlife Trust

regular surveying of river life in the stream

Image: H & IOW Wildlife Trust

Riverfly & Dissolved Oxygen Monitoring

The monitoring of riverflies and other aquatic insects is a very good indicator of sewage impact.

Their numbers can be affected by algae smothering their eggs. There will be a lower diversity of insects than would be typical for this type of stream and the relative absence of more sensitive species such as the Blue-winged Olive and an increased presence of the more robust water water louse are tell-tale signs.

Survey results for the chalk stream have been below the 'trigger level' where a report is sent to the Environment Agency four times in the past year.

river insect life we find in our chalk stream

Image: H & IOW Wildlife Trust

we all have off-mains septic systems in the village

Image: H & IOW Wildlife Trust

Our Legal Responsibilities

We have a legal responsibility for keeping our systems healthy and can be prosecuted if not. Off- Mains systems are regulated by the General Binding Rules which states:

You must empty your systems regularly according to the recommendations of the manufacturer or specialist wet waste companies who can advise based on size, system type, and number people using it. In many cases, this would be at least once a year. The myth that tanks do no not need to be emptied is just that - a myth - and breaches regulations.

You must service your systems and repair faults so that they are functioning properly.

Systems in certain locations or those with high-volume discharges may need a permit.

It is illegal for septic systems to drain directly into a water course

In all cases the bottom line is that discharges must not cause pollution.

For a full version of the rules, please visit

regular servicing of systems is a legal responsibility

Image: H & IOW Wildlife Trust

Bishop's Sutton Parish Council are partnering with Hampshire & Isle Of Wight Wildlife Trust and Watercress & Winterbournes for the chalk stream initiatives. Thanks go to Sophie Evingar at H & IOW Wildlife Trust and Ian Diver at Watercress & Winterbourne for their kind permission to use their information and images for this website. 

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