The Norwegian's developed the ...

The Norwegian's developed the MBBR process as an inexpensive means of upgrading and increasing the treatment capacity of existing wastewater treatment plants that had absolutely no room for expansion - many being located between the base of a cliff and the ocean.  There are other hybrid technologies and MBR processes you could consider, but I believe from an overall sustainability basis the MBBR is likely your best choice.  It is very simple to operate; has lower aeration energy costs than a MBR process and no membranes to replace; it does not have the sludge bulking problems experienced at conventional activated sludge processes; capacity can be expanded and reduced to accommodate variable loading by adding or removing media; the waste biosolids typically settle and dewater well; and the inherently long sludge ages minimize biosolids treatment and disposal costs; and there is a great deal of price competition in sourcing media.