Just a few words on the cattle slurry situation.
There are a 1,001 factors involved but I'll keep it simple. Ireland has an excellent climate for growing grass so we do, and we feed that grass to dairy and beef cattle. Grass is not nutrient dense so the animals eat a lot of it and therefore produce a lot of poop.
That poop is rich in microbes, in fact it is mostly microbes with some undigested fibre (lignin and cellulose) mixed in, along with a lot of water.
A lot of nitrogen that is fed to the grass to make it grow ends up in the poop. Mixing all that nitrogen into a watercourse will cause oxygen deprivation due to aerobic microbes feeding on it. Milk, BTW, has a far higher biological oxygen demand and is a far worse pollutant.
It is this lack of oxygen in the water that kills the fish.
Farmers are therefore obliged to collect the poop and dispose of it without it getting into watercourses. It is an excellent fertiliser and so spreading it back on the land is the best thing to do with it. This has been known since farming began.
However, it does need storing over the winter when cattle are housed and, TBH, most farmers haven't enough storage although they meet the legal requirements. When the ground is wet the slurry can run off the the fields into the rivers, but farmers run out of storage and have put it out just as soon as they legally can although conditions might not be suitable.
There is a general lack of storage because herd sizes have increased but not the facilities to deal with the waste. Slurry separation can help, but more storage is the real answer.
While in storage the microbes continue to break down the remaining solids and produce methane and other gasses in doing so. This mixture can be collected, purified and used as a fuel, and Agriland has covered this in the past, most farming media has.
However, it does take a big investment and the returns are variable so it has not caught on as much as certain interested parties might hope. New Holland are looking into it to fuel tractors and have pilot plants over in Cornwall, but it's motives might not be quite what they appear.
Central ADs (Anaerobic digestion plants) are a nonsensical idea as it means transporting crap miles rather than using it at source, and it is mainly water, and heaving water about pointlessly is not green by any standard.
That's the bones of it, but there is an awful lot of detail involved.