Disinfecting drinking water for personal use is not hard to do, but it depends on how much you need and how permanent the problem is! Disinfecting a cup of water on a camping trip is not the same as trying to process the flow from a spring supplying your entire house, and it eventually comes down to cost and power.
Boiling water
The bacteria (and almost all other pathogens likely to be found in rivers and lakes) are killed by boiling the water for a few minutes – so this works best on small volumes that you wanted hot to begin with. Boiling is the first choice for camping trips and also means that food cooked at or above boiling point (such as meat) will also be rendered safe in the process. It of course needs a power source, but it doesn’t involve chemcials so is acceptable to people wanting the least impact on their diet. Boiling is not an option for a general household water supply, but of course makes boiling water for hot drinks safe.
Iodine tablets
Water purification tablets that use iodine are very effective too, and don’t need a heat source, but can leave a taste that some people find unpleasant. They are however easy to use and light to carry, so are ideal for long hikes and expeditions to remote areas. It’s not an option for large volumes or a household supply.
UV sterilisers
These are also very effective against leptospira (and most other pathogens), and can be bought on all scales. Permanent systems for processing flowing water in a domestic house are available, but can be expensive and use electricity. Portable devices for camping and backpacking are also available, resembling a small flashlight or pen, and these can be used to sterilise a single cup of water – but need batteries. The advantage of using UV is that there are no chemicals involved, and the electricity required is less than for boiling the same volume. It does require the sample to be transparent though, so can’t be used on things like milk.
Chlorination
Water not used for drinking – such as swimming pools and ornamental features, can be protected by adding dissolved chlorine. In commercial pools this is a complex engineering process, but for private pools it’s usually added in the form of tablets. It’s important to keep the chlorine levels high enough to be effective but not too high, so testing kits (using color-changing paper) are available. Leptospirosis is killed by the concentrations of chlorine used in commercial pools and most private ones, assuming the owner follows the instructions on the packet!
Filtration – does not work!
Leptospira are incredibly small – they can pass through the pores in water filters, even those that claim to remove bacteria. They will pass easily through filters with a pore size of more than 0.2 micron, including membrane and charcoal types. High-pressure laboratory filters with a pore size of less than 0.1 micron will block them, but the typical hand-held water filters used by hikers, pool filters and the fitted cannisters used in some domestic kitchen applicanes are useless at removing leptospires – they are often used to separate leptospires from other bacteria when preparing samples for research, as the leptospires pass through but other bacteria don’t!