If you have suspended solids ...

If you have suspended solids going into your MBR a better screen or filter would be a cheap option to reduce loading and increase your effluent quality. After the MBR if needed an AC filter is a good option but bear in mind the cost of getting the filter recharged. Minimise the loading on your RO anyway you can, less energy and less fouling.  RO will probably be your most expensive process in terms of kg nutrient removed, depending on the application and effluent quality requirements you could RO a fraction of your flow and dilute. Lots of options but I'd say start from the beginning and look for any opportunity to reduce loading.