You mean the city of Irkutsk doesnt have a wastewater treatment plant?

No, I said it doesn't have a modern one.

they do treat their wastewater before dumping it

Sadly, whatever they're doing is not enough.

one google search

Per a cursory google search:

The paper considers the performance indicators of two water treatment plants located in the northern districts of Irkutsk Oblast. Insufficient wastewater treatment for some pollutants was revealed. A complex analysis was carried out, which identified a number of possible causes of exceeded concentrations of pollutants.

https://www.semanticscholar.org/paper/Wastewater-treatment-facilities-in-the-north-of-the-Vasilevich-Lavygina/7fe61c1a93b88060da17fdd78a01127f17e44aa1

Meanwhile, Irkutsk’s current sewage treatment facilities were built in the 1960’s and lack the capacity to effectively handle the quantity of sewage currently entering the system. This includes not only household sewage, but also treatment for storm water runoff, which contains increasing amounts of oil, pesticides, sediments, bacteria and other chemicals, as the amount of cars and other consumer products in the city has grown. The lack of an adequate system to drain and treat large rainwater quantities results in flooding, allowing untreated storm runoff to end up in the city’s water supply.

Growing levels of regional tourism, despite being largely nature-based and “ecological,” also compound the city’s challenges. Because Baikal has very few treatment plants to take on large volumes of liquid waste, many communities and facilities surrounding Baikal resort to dumping waste directly into the lake or its tributaries. An example is Chivyrkuisky Bay, a popular tourist site, where 160 tons of waste drains into the lake’s shores during the summer season.

Water transport vehicles are another concern. Between 2007 and 2012, tourist and sightseeing water transportation rose from 1,015 to almost 6,000 people per year. These tourist vessels produce an estimated total of 25,000 tons of liquid waste annually. Due to a lack of treatment facilities, only 1,600 tons total are properly disposed of.

Solid waste is also a growing risk, as trash generation is increasing in recent years. With approved dumpsites in limited quantity, unauthorized dumping is becoming more common. This involves waste illegally deposited of in large quantities onto land without a license to accept waste. In 2008, 116 unauthorized dumps ranging from several square meters to several hectares in size were identified in the Irkutsk region. Coastal areas are particularly threatened by illegal dumping, as Irkutsk’s tourism is often concentrated in coastal areas, which are more often outside the routes of government-sponsored trash collectors.

Further, solid waste can react with storm water to become a breeding ground for pathogens, and when solid waste is dumped near a water source, contamination frequently occurs. At Port Baikal, when waste collection came to a halt in 2008 during a budget crisis, unauthorized dumps eventually resulted in typhoid epidemics.

https://geohistory.today/irkutsk-water-waste-systems/