With regard to the part of your question that asks about "piping" (that I notice has not yet been answered for more than a week), it is my understanding a great deal of ductile iron piping is used for sludge and scum piping, and particularly in the larger wastewater treatment plants in the USA (some of which have access to many decades of varied operational experience). I believe there are perhaps many reasons for this, and among these is the fact that, "Grease has a tendency to coat the inside of piping used for transporting primary sludge and scum." ("Wastewater Engineering", Metcalf & Eddy, 3rd edition, 1991).
Where grease or other similar build-ups are expected to be especially problematic in process operations, ductile iron piping additionally has the capability to be furnished with "glass-lining". I have heard at least anecdotally many times over the years that there are less blockages with such contact material system (compared to other, more oleophilic/or having affinity for oils/grease pipe materials and linings, such as plastics or polymers etc.) Additionally, per the same reference I mentioned before, "In some plants, provisions have been made for melting the grease by circulating hot water, steam, or digester supernatant through the main sludge lines," and ductile iron/lining systems are known available for withstanding effects of the injected heat and other maintenance operations if this were necessary.
I suspect for similar reasons most lines of this nature are also perhaps most typically of 6”-8" or larger sizes, that generally I suspect are more conducive to conventional mechanical cleaning operations than smaller pipes.
Further to the comments on pumps from stanier, I noticed the aforesaid reference (and probably many others) incidentally also talks about pumping many quite variable natures of sludges with many different specific kinds of pumps, and then also says, "Diaphram and centrifugal pumps are used extensively for pumping scum."
Like you, however, I will watch for more knowledgeable or direct experience responses.
For scum, you can use the Wemco Model CF (Chopper pump if you have large size materials), the Wemco Model E (a non clog recessed impeller type of pump), or the Wemco Hidrostal.
Piping is typically of 6”-8" ductile iron with provision for cleaning or flushing.
