THE MAGAZINE FOR THE DRAINAGE, WATER & WASTEWATER INDUSTRIES
ENVIRONMENTAL NEWS
Poole Harbour, Dorset, UK an EA Nutrient Neutrality site
improved PTPs, which may generate nutrient credits. For example, an applicant proposing new houses might be able to replace septic tanks at existing neighbouring properties to bring the whole area into line with requirements, provided the units are appropriately located.
Failing systems should not be able to claim nutrient credits beyond the default baseline for expected performance, meaning that poorly performing systems should not be rewarded. Additionally:
are qualified service engineers where, if dosing is required, users must take out a maintenance contract with an approved company, this is not the case in the UK.
In the UK, chemical dosing in off-mains drainage systems is not currently advanced enough to precisely deliver chemicals, and it is likely that underdosing or overdosing will occur. Furthermore, unlike the utilities, domestic discharges lack environmental consents for parameters such as iron or aluminium, which could have a negative impact on receiving waters.
For example, water pollution from the chemicals used in domestic wastewater treatment, such as ferric chloride, aluminium sulphate, and ferric sulphate, through overdosing or accidental release into the environment can contaminate ecosystems, harming wildlife and disrupting natural processes, causes algal blooms, depleting oxygen, discolouration of the stream or watercourse and killing aquatic life.
While in small quantities this may not be harmful, over time the pH level in water bodies can be affected as well as being harmful to those handling chemicals if they are not trained professionals.
The most recent updated guidance given by Natural England in 2022 states, “The use of PTPs that include chemical dosing on privately owned properties, which are not controlled by a
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management company or similar, should not be used to achieve nutrient neutrality. This is unless the local planning authority is satisfied that sufficient measures have been secured to ensure that the correct dosing will be applied by the homeowner in perpetuity.”
Effective mitigation
While nutrient neutrality is complex and the guidance under constant review, some effective mitigation schemes have taken place to the benefit of developers, and WCSEE is confident of further technological advancements emerging soon.
Natural England guidance for domestic package treatment plants state that where deployment is appropriate, a plant will not necessarily deliver nutrient neutrality unless it is replacing an inefficient septic tank or PTP, or the existing nutrient export from the developed land is high enough to completely offset the PTP nutrient load.
In most cases a PTP will need to be delivered in conjunction with other mitigation measures, either within the development or offsite, to achieve nutrient neutrality.
Upgrading PTPs
For existing off-main drainage properties within nutrient neutrality areas, the guidance is to replace existing inefficient septic tanks and PTPs with
PTPs or septic tanks that discharge to ground should only be replaced by units that also discharge to ground where ground conditions are appropriate for drainage. Existing units that currently discharge to water may be replaced by units that discharge either directly to water or to ground.
All PTP and septic tank replacement schemes will need to meet the Environment Agency permit requirements.
The replacement of existing units which meet the threshold for small-scale discharges of less than 2m3/day cannot be used to generate phosphorus credits.
Anyone looking to install a small onsite PTP should read Natural England’s guidance carefully, liaise with their LPA, and contact WCSEE for clear technical advice about off-mains drainage and wastewater treatment solutions.
Site Site-specific chemical dosing PTP solutions already exist in the market and, if an off mains drainage property is in a nutrient neutrality area, can form part of a suite of solutions to achieve phosphorus offsets. For example, WCSEE is developing an algae-based system for the Diamond range of PTPs, backed by comprehensive research and three years of data which show up to 80% phosphorous removal.
Over time, the expectation is that these biologically based solutions become more commonplace in the market, so domestic properties with off-mains drainage have access to a range of non- chemical solutions from a variety of manufacturers to the ultimate benefit of the environment.
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