5.3.6 Effluent disposal

a.         Extent of the activity

Within and adjacent to the cSAC there are a number of coastal discharges, the majority of which are wastewater treatment works, treating domestic sewage.

i.   Within the site

Effluent disposal points are found throughout the area of Cardigan Bay. Those that appear in, Table 9. , Table 10. & Table 11. discharge into coastal, estuarial or freshwaters within or immediately adjacent to the site. Although Cardigan STW, its associated discharges, and two sites at Aberporth , are just outside the cSAC boundary, they have been included here because they serve a relatively large part of the local population and are extremely close to the site boundary. DCWW sewage effluent discharges are shown in Table 9. , which also describes treatment processes, showing both the existing and planned treatment. All sites are consented by Environment Agency Wales. See Figure 17.

Table 9.   Dwr Cymru Welsh Water Waste Water Treatment Works

Types of treatment:

PRIMARY TREATMENT: The purpose of primary treatment is to remove from the sewage much of the readily settleable material, such that the subsequent biological treatment is not overloaded. This is achieved in continuous flow tanks where the velocity of the sewage is reduced so that solids settle out and can be removed as sludge. The top liquor or ‘settled sewage’ passes forward to secondary treatment.

SECONDARY TREATMENT: Secondary treatment comprises biological oxidation of sewage by either (a) biological filtration (fixed bed reactor) followed by settlement of humus, or (b) the activated sludge process (suspended biomass reactor) followed by settlement of the sludge and separation of the surplus. The basic requirements for the viability of both processes are a food supply (the sewage), suitable organisms and an adequate supply of oxygen.

TERTIARY TREATMENT: Additional stages of treatment to further polish the effluent include (a) filtration, (b) nutrient reduction, (c) phosphorus removal (d) reed beds, (e) microfiltration and (f) ultra violet disinfection.

There are also a number of overflows connected to sewage pumping stations (SPSs) in the area, which are designed to operate only in cases of heavy rainfall in order to relieve pressure on the system and prevent sewage from flooding local properties. There are also a number of combined sewer overflows (CSOs) within the area. These act as relief valves, designed to discharge diluted sewage during periods of heavy rainfall when the receiving watercourse provides high dilution. These overflows may contain both domestic and trade effluent and storm water. Details of these two types of intermittent discharges are shown in Table 9.

Figure 19.   Effluent treatment, transfer and disposal sites in and adjacent to Cardigan Bay cSAC

Table 10.  Dwr Cymru Welsh Water - Intermittent Discharges

There are also a number of discharges of sewage from SPSs within the area, which discharge only in the case of an emergency. These are shown in Table 11.

Table 11.  Dwr Cymru Welsh Water SPS Emergency Discharges

Most industrial premises in the area connect to sewers passing to DCWW STWs. There are also a number of small STWs belonging to the local authority and others, many of which serve holiday developments. In addition to these, there is one industrial site which discharges into the cSAC, Quay Fresh and Frozen Foods at New Quay. All sites are consented and monitored by Environment Agency Wales.

ii.  Adjacent to the site

As well as effluent disposal points within the cSAC itself, it is necessary to consider disposal points beyond the site boundary which may have an influence on the water quality of the site. There are numerous small STWs and some industrial sites which discharge effluent into rivers flowing directly into the cSAC, such as the Teifi and the Aeron. Relative inputs from these are monitored directly, and by monitoring the quality of the rivers into which they flow.

It is also possible, given movements of tidal waters, that discharges well beyond the cSAC boundary may influence the quality of water within the site. However, it is obviously not feasible to examine every discharge at this stage, but investigations into any water quality problems experienced within the cSAC will not be limited to those sites listed above, but to any potential source.

b.   Mechanism for effect on the feature

It is possible that human and domestic stock pathogens can enter the sea via sewage treatment works discharges since no disinfection processes are absolute, and many viruses can survive a very long time, particularly in sediments. There is the potential for infection of bottlenose dolphins by some of these. See ‘Pathogens’. Chemical inputs through effluent disposal can also affect bottlenose dolphins. See ‘Pollution’.

c.    Management Response

i.   Rationale

All discharges are controlled through a statutory system of licences know as discharge consents, issued by Environment Agency Wales.

No epidemic mortality of dolphins to a specific human or terrestrial animal pathogen has been recorded. Dilution of discharges is very high, even in low tidal energy environments such as Cardigan Bay, and there is no obvious mechanism for delivering a “challenge dose” of sufficient magnitude for a pathogen to cross a species barrier.

There is currently no evidence to support the idea that chemical quality from sewage and trade discharges, and sewage derived viral and bacterial pathogens, pose a significant risk to the Cardigan Bay dolphins per se. It is more realistic to expect pathogen mediated mortality to arise from impairment of the immune system occasioned by accumulation of immunosuppressive contaminants (possibly PCBs). All animals carry a significant burden of microbiological pathogens around with them all the time, and it is to these that they are most likely to succumb when resistance to disease is lowered.

Complete removal of all human and stock derived bacteria and viruses from marine inputs is not an option. Other sewage related issues, such as aesthetics, are being dealt with under current DCWW’s Asset Management Plan (AMP) investment arrangements.

Although these discharges are not thought to currently pose a significant risk, the Habitats Directive requires a review of relevant consents and permissions. See ‘Plans and Projects’.

ii.  Type of Response

F1 : The activity constitutes a plan or project

F4 : There is a known mechanism for the activity to have an effect, but there is insufficient evidence at present to determine whether or not it is having a significant adverse effect.

iii.    Actions, including links to other policies/plans/measures