Would it be worth having a pre filter system with some brushes/filter foam to assist with the poo issue. I am defo going to put in a diy sieve if the top pond goes ahead so that I assume will filter that waste. I will have to be carefull that I don't get too much of a restriction for the pump though but it will help. Could I perhaps put in a more technical sieve prior to the pump? Like the nexus?
The subject of mechanical filtration I think is the most diverse in ponds. Many different approaches. Mechanical is also normally a multi stage type thing where you're trying to remove smaller and smaller bits at each stage while trying to also make maintenance easier. That makes it complex.
Pre-filter
Current state of the art imo is BD gravity fed sieve filter and then the pump. That should reduce 95% of the waste (assuming a good skimmer system and TPR system). It's effectiveness isn't just that it traps stuff but that it removes the waste from the water column automatically 24/7 and that it's easy to clean. These are very important factors. A filter that is difficult or expensive to clean stops being cleaned and becomes worthless. This was one factor in the decline of bead filters as many people had to clean them once a day. At 100-200 gals a day it became expensive to back wash compared to other choices.
Instead of a sieve there are many choices that I don't think really ever performed very well. Drum filters, end up clogging with bio film and it seems like they get used for a few years and then tossed. Settling tanks have to be large and the back washing cost of water adds up so these often get tossed after a few years.
Polishing
After the sieve the choices depend more on your goals. You could just pump to bio, UV and back to pond. Or you can add more mechanical like the bead. Bead filters for this use depend on the bead size. Smaller the bead, smaller the gaps between beads, smaller the particles trapped. Bigger the bead the less trapped particles but the better it works as a bio filter. But currently few people depend on bead filters for the bio...just not as good as other options. I'd say the current state of the art for removing small particles is a sand and gravel filter. These are back washed just like the bead. The only real difference between SG and bead filters is the size of the "beads". Sand being tiny can trap lots more stuff resulting in clearer water. Depends on you're goal. For many ponds, especially water gardens super clean water isn't needed to have acceptable clear water. Plants cover much of the pond so you may not see a lot of water. A 2' deep pond can look perfectly clear while that same water in a 5' deep pond may appear murky. The more particles you have to look thru the less clear the water appears. People often put pond water in a glass and see it's perfectly clear, like tap water, but wonder why their pond isn't clear.
DOC
The final stage for mechanical filtering is a foam fractionator, also called a protein skimmer. Here you're removing particles that are smaller than bacteria (DOCs). At this stage these are the state of the art and the only other options are chemical, like flocculation. There are different foam fractionator designs for getting more particles out but even a super simple design can be very cheap, easy to build, effective and 100% maintenance free.
Bog
Bogs cna be used as pre-filter and/or polishing filter. As a pre-filter the downside is it can fill quickly with muck and cleaning is difficult. As a polishing filter they act as a settlement chamber for small particles, but not very small particles. The down side is waste remains in the pond and continues to decompose which will add to the DOC level.
Main choice.
The main choice is whether to spend time during the design phase learning and designing or spend time later cleaning filters, changing stuff which can be difficult after the build. Most people seem to prefer the later on their first pond or two and the former on subsequent builds.
Disclaimer
Understand that I don't think BDs and mechanical filters are a requirement or even a plus on all ponds. Depends on the pond and the owner's goals. You mentioned BDs and so I'm just talking about BDs. I don't know anything about your design or goals, too few details given. I really don't need to spawn more threads on how WB is wrong to think every pond requires a bunch of complex mechanical filters.
