When you filter liters upon liters of water, it leaves you with a massive filter and you can't possibly process the whole thing. Instead, you end up cutting out a tiny portion and using that, which is no different than just using a smaller filter.
These types of filtering methods also often require open filtering and reusing equipment, which is susceptible to contamination, as well as bleach for disinfecting, which can leave residues that degrade DNA.
We have designed our sampling protocol to be easy enough for anyone to do, while still being free of contamination and highly effective. Instead of a massive filter, the entire 22mm syringe filter can be extracted. We run replicates to try and capture as much DNA from the filter. We also recommend that you take more samples in the field to increase the chances of capturing the DNA you're trying to find.