The positive effects of disinfection show throughout Porvoonjoki, from the wastewater discharge area all the way down to the estuary. When UV disinfection is in use, the average quality of the water in Porvoonjoki fulfils the requirements for irrigation water from Orimattila to the estuary.
Basically, thanks to UV disinfection, the quality of swimming water in the lower parts of the river also remains good from spring to autumn. The above information is based on a recent survey carried out by Itä-Uudenmaan ja Porvoonjoen vesien- ja ilmansuojeluyhdistys (Water and Air Protection Association for Eastern Uusimaa and Porvoonjoki River), which investigated the effects of UV disinfection on hygienic water quality in Porvoonjoki.
The UV disinfection of wastewater from Lahti reduces the concentrations of faecal Escherichia coli and Enterococcus bacteria in the upper reaches of the river by more than 80% compared to no UV treatment. The influence of disinfection also shows in the lower reaches where the levels of bacteria have decreased by more than 60%. This means that for bacteria, the overall wastewater purification efficiency in the whole process is more than 99.9% when compared with the amount of non-purified wastewater entering the treatment plants.
Positive effects on bodies of water
UV disinfection was introduced at Lahti Aqua's wastewater treatment plants in Ali-Juhakkala and Kariniemi in 2015. The hygienisation plant, based on wastewater UV treatment, was among the largest in the Nordic countries. Corresponding treatment was adopted in a wastewater treatment plant in Nastola in 2017. UV light changes faecal bacteria so that it cannot reproduce or cause infections. A total of 12.8 million m3 of wastewater is treated in the plants every year. According to environmental permit conditions, UV disinfection remains in use from early spring until late autumn.
The above survey was the first one conducted in Finland and indicates the positive effects of UV disinfection on bodies of water. It shows that investing in wastewater UV disinfection will considerably improve the hygienic quality of the receiving bodies of water. The UV-based after-treatment of purified wastewater will be introduced in different parts of Finland in the next few years.
Itä-Uudenmaan ja Porvoonjoen vesien- ja ilmansuojeluyhdistys ry
• Executive director, limnologist Juha Niemi tel. +358 50 571 0335, email@example.com
• Senior researcher, biologist Mikael Henriksson tel. +358 50 490 0692, firstname.lastname@example.org
Lahti Aqua Oy
• Technical director Jouni Lillman tel. +358 3 851 5932, email@example.com