Photos at Tywyn sewage treatment works have sparked fresh fears about pollution following aerial images of discoloured seas, fish deaths, swimmers becoming ill and an incursion of foul-smelling algae.

Photographs taken on a long-lens camera by lifelong surfer Robert Turner appear to show what he described as sewage tankers being deposited into the treatment plant while others were discharged ‘suspiciously’ into an unknown drain – prompting fears over raw effluent outflow into the sea.

A spokesperson for the nation’s primary water supplier, Dŵr Cymru, said the images show workers depositing sewage into an inlet and not Tywyn Main Drain – the pipe shown in the aerial images published last week, from which brown discolouration appears to be originating. A spokesman from environmental regulator Natural Resources Wales (NRW), which is responsible for the main drain, repeated that all tests carried out so far have not recorded the presence of sewage or anything toxic.

NRW and Dŵr Cymru say there have been no unpermitted discharge and that all extra sewage being transported from Dolgellau wastewater treatment works – currently closed for essential maintenance – is being treated and then compliantly released. NRW reports that the emergency outflow, which leads into the main drain, has never been used and Dŵr Cymru says the Combined Sewage Overflow (CSO) drain, for use only when the system is under extreme pressure, was last activated in December.

But NRW appears only to have tested the bathing water on 23 May, when residents say the brown slick – which smelled like and resembled sewage – appeared on 28 May. Former councillor Mike Stevens told the Cambrian News he has sent off his own tests and dismissed the credibility of NRW’s if they were so long before the thick brown algal scum was at its worst.

This follows photographs provided by Snowdonia Flight School showing brown seawater next to the town’s treatment works which residents say call into question environmental regulator Natural Resources Wales’ (NRW) assertions that the ‘unprecedentedly’ smelly and thick algal bloom - which weeks ago overwhelmed large stretches of Cardigan Bay – is non-toxic and unrelated to pollution.

Dŵr Cymru added that the plant can treat up to 58 litres per second before capacity issues – and is currently treating only 20 l/s due to the prolonged dry weather - and despite the extra sewage from Dolgellau.

But residents and a former environmental protection officer are doubtful that, at the end of last month, there was no discharge which intensified the blooming algae. They say it could have come from the main drain but also fear Dŵr Cymru is disguising discharges by releasing directly into it through the emergency drain, rather than through the monitored CSO drain – which discharges 1.5km out at sea.

Residents are still not reassured after Tywyn caravan owner Clive Birch’s son went swimming with his friends who then became ill, Aberdyfi chartered sea-angling skipper Charlie Bartlett said there were very few fish over the reefs and he’d never seen an algal bloom so bad in 54 years, and anglers reported and photographed dead fish, effluent and toilet paper.

In good news for residents, Dŵr Cymru confirmed in a meeting with Mr Stevens that the transportation of the sewage tankers from Dolgellau to Tywyn will cease now in July, rather than in late Autumn as originally planned.

Dŵr Cymru said that, at peak, 16 tankers of sewage were being transported to Tywyn per day but significantly fewer are arriving at the plant now. Mr Turner said he saw five come and go from the site in little over two hours.

Mr Turner, who said he had suffered diarrhoea after swimming recently, added: “My wife relies on the sea for her mental health, and she hasn’t been able to go in now for two weeks because the water quality is so bad. You go in the sea, and you stink. We’ve been advising people not to go in – because we knew people who were getting ill. I’ve known Tywyn my whole life and lived in Cornwall for 20 years, and I’ve never seen an algae bloom like this.”

He said whatever is coming out of the treatment works is ‘unbalancing nature’.

Mr Stevens told the Cambrian News: “I talked to an NRW officer on 6 June and he said they tested 24 May and 30 May. I asked did they actually test the large brown slick on 28 May, and he said no. I was contacted again on 6 June as there was chocolate sludge water at the Neptune Road end of the beach. I have taken samples and sent them to Swim Safe for analysis.”

An NRW spokesperson said: “Algal blooms are seasonal events and the recent weather has been ideal for algal blooms to grow while tide and wind conditions have kept them close to shore. The Tywyn Main Drain receives water from the surrounding low-lying land close to the Dysynni Estuary. This was inspected by NRW staff on Friday, 9 June and we have no concerns about the watercourse.

“Dŵr Cymru states the Tywyn site is operating within its limits and we are continuing to carry out our regulatory checks. We have had no recent reports of incidents from Dŵr Cymru Welsh Water or storming at Tywyn. As part of our regulatory work, NRW visited the works and performed a number of checks during tankering operations, and no issues were found.”