There are generally two ways to fix water pollution in Cities: Skylines. You can actively reduce water pollution by using structures that process and clean up waste, or you can remove the sources of pollution and then let nature take its course. After a long while, the pollution levels should return to normal. Of course, you can use both at the same time, i.e. stop the source and add structures that will actively reduce existing pollution to speed up the natural cleaning process.
To better understand these methods, you need to first understand how water pollution actually works in Cities: Skylines.
Water Pollution in Cities: Skylines
Water pollution in Cities: Skylines is primarily caused by sewage, and to a lesser degree, industrial work (or other pollution-producing structures) being too close to bodies of water. Prevention is better than cure, so it’s ideal that when zoning, you paint industrial zones away from bodies of water. Additionally, some buildings like garbage landfills and coal power plants indicate a sound pollution and waste pollution radius—if any bodies of water are within the waste pollution radius, they will eventually become polluted.
While being strategic with building placement and industry zoning is nice, the primary cause of water pollution in Cities: Skylines is nigh inescapable—sewage. In the game, you typically use a water pumping station to draw from a source like a river, cycle it through your city via water pipes, and dump it back out into the water in the form of sewage. The best approach to this is one the game makes clear from the onset via tooltips and building descriptors: place your sewage outlets downstream from the water sucked up by your water intake, or you’re gonna be giving your citizens polluted water. Polluted water sources naturally lead to problems like sickness and complaints, not to mention decreased land value for zones near the pollution.
An Important Note on Prevention
If you’re reading this guide, then you already have a problem with water pollution. It’s probably making your citizens sick or negatively impacting your natural fish population, which in turn hurts your fishing industry. But in case you’re reading this to AVOID problems in the future, then you should know that the quickest way to prevent water pollution is not to use water drain pipes on bodies of water at all.
Use water towers and inland water treatment plants so you don’t even have to touch bodies of water for your city’s needs. Note, however, that a water pumping station pumps 120,000 cubic meters of water a week, whereas the starting water tower only pumps 60,000 cubic meters of water a week, and is slightly more expensive to build. This means that when starting out, it’s more cost effective to use a water pumping station. On the other hand, there’s practically no difference to the cost, maintenance, and drainage capabilities between water drain pipes that spill sewage into bodies of water, and inland water treatment plants that drain waste water back into the ground.
So if you want to completely avoid polluting your bodies of water, you can paint and build all pollution-producing industries and buildings away from them, and use inland water treatment plants instead of water drain pipes, again taking their pollution radius into consideration.
Fix #1: Strategically Move Your Drain Pipes Around
Unless you’re playing using mods that allow for unlimited money or fully unlocked progression milestones that make every building available, your options are going to be limited by your budget and your city’s rate of growth. So if a certain area of water is getting too polluted and you need to address it without spending too much, you can simply reposition your water drain pipes to a new area. Repositioning costs less than building new structures.
As mentioned earlier, once the source of the pollution disappears, the body of water will naturally recover over time. That said, simply moving the drain pipes around will of course just also move the pollution around, but it’s a good option if you’re low on funds or if you’re waiting for your city to grow to a point when you could invest in the upgraded, eco-versions of drain pipes or treatment plants, as those are more efficient in reducing waste in sewage.
Fix #2: Use Water Treatment Plants
Water treatment plants function the same way as water drain pipes, but reduce water pollution by a whopping 85%. It’s the first significantly meaningful fix for water pollution that unlocks once your city becomes a “Big City,” costs six times more than a simple water drain pipe, and costs more money and power to maintain. It is, however, worth all that if your city is suffering from a massive water pollution problem—until you unlock its eco-friendly version, as you’ll see next.
Fix #3: Use Upgraded, Eco-Friendly Options
If you got the money and the options are unlocked, you could invest in eco-friendly upgrades. The eco water outlet, for example, produces less pollution compared to its non-eco counterpart, the basic water drain pipes. However, it costs nearly twice as much to build and drains half the cubic meter capacity per week. Eco water outlets are immediately available though, just like simple water drain pipes, so it’s a good investment if you think water pollution may be an issue later.
The water treatment plant also has an eco-version that actually flushes out 94% of waste in sewage, but is only available once your city grows into a “Capital City,” and is also around 60% more expensive to build. Interestingly, it costs less to maintain, though its power consumption is over thrice as much as its non-eco counterpart.
Fix #4: Use Floating Garbage Collectors
Floating garbage collectors filter waste from the water and speeds up the natural recovery process. It’s called a floating garbage collector but is primarily used to cleanse water pollution.
You can cluster a few of these together for maximum effect, but note that a single unit comes at a costly 50,000 credits. Once it’s done cleansing an area, you can move it to another polluted area to work its magic there.
Fix #5: Use Inland Water Treatment Plants
As mentioned previously, you can avoid dumping sewage into bodies of water by using inland draining options, and both the inland water treatment plant and its eco-version are immediately available at the start of a game. More advanced versions with better capabilities are available at later stages.
Additional Tips for Dealing with Water Pollution in Cities: Skylines
There are a few other things you can do to not only fix water pollution, but make the most out of the situation if it arises.
Cluster Pollution Sources
Make sure you cluster as many sources of pollution as close together as possible. Place water drain pipes close to each other, for example. This way, though it generates more significant pollution, it’s easier to place structures that fix the pollution later.
Bottleneck Pollution Downstream
Depending on the geography of your map, you can bottleneck downstream water pollution to make it easier to fix by placing the appropriate structures like floating garbage collectors where all the pollution is concentrated.
In the screenshot above, a few concepts are used together: clustering sources of pollution together, using a bottleneck to direct pollution into an easily addressed area, and making use of the polluted water for other purposes, as you’ll see below.
Make Use of Polluted Water Anyway
Not for drinking, of course, but for other uses. For example, a hydro power plant that uses a dam generates power from the speed of water flow, and it doesn’t really matter if the water is polluted. The same goes for an ocean thermal energy conversion plant that uses water temperature. This way, areas of polluted water still have some meaningful use.