At Malmesbury, Wiltshire, visitors were waiting at the entrance when the gates of the small works opened at 4.30pm. They kept coming - retired couples, parents with children, besuited businessmen straight from the office.
They took a half-hour guided tour and duly appreciated the contrast between the grey- brown foul water pouring into the works and the odourless, almost clear liquid flowing steadily into the River Avon.
Wessex Water, second smallest of England and Wales' big 10 water companies, has opened a fifth of its 360 works this week and been flattered by customers' response. It expects peak numbers at the weekend.
Water bills have been rising above inflation for several years and will continue to do so. Improvements in sewers and sewage treatment accounts for more than half of the increase. Libby Gawith, Wessex's divisional sewage manager in charge of 124 treatment works and 100 staff, hopes customers will find the bills more acceptable if they understand what has to done.
'They expect to find a kind of chemical works,' said Ms Gawith. 'They don't realise that a sewage works is a living thing which requires sensitive handling. There are old hands who can walk in the gates, take one sniff and realise the place isn't running properly.'
What most fascinated visitors were the round filter beds filled with volcanic rocks and the real workforce - a diverse community of bacteria, protozoa, worms and other small creatures digesting the sewage and each other.
The Malmesbury works lies among rolling meadows and tall hedges. What little odour strikes the visitor on entry soon goes unnoticed - apart from the ghastly stench encountered above a tank of untreated sludge.
Ms Gawith hopes sewage week will improve the self-esteem of her staff in what can be a lonely and unregarded job. But Geoff West, who retires next year after 16 years at several local works, has no complaints. 'I'll be disappointed to leave,' he said. 'You see the seasons come and go, you see all the wildlife . . . I reckon it's great.'