Having been a sludge process ...

Having been a sludge process scientist some years ago I have always kept a keen eye on sludge tech as conventional wastewater treatment produces endless quantities of the stuff.  The basic rule of thumb is the more you treat wastewater then the more sludge you produce, giving you an even bigger headache in dealing with it.  Like everything in wastewater the options available are dependent what restrictions are placed on the Company by legislation, how much money you have available to spend and what (if any) your target market is for the end product.  

Here in the UK if you want to use the sludge as a product suitable for agriculture, then you must treat it to a high standard by reducing pathogens.  The US has a similar scheme.  Keeping pathogens down mean that only certain technology can deliver this outcome.  Anaerobic digestion  provides a lower quality product which (in general) needs to be dewatered before application, but the rush for feed in tariffs have made this technology very popular.  To reduce the pathogen numbers more then you can go for cambi technology (a pressure cooker type technology), sludge drying, but with experience this technology has receded greatly from what was seen as the "ultimate solution".  Less capital intensive technology are the lime technologies, which requires less capital expenditure, but is expensive to run and a right pain H&S wise from using quick lime.  I would add that when deciding on what sludge technology you want you must factor in how to treat the liquids that will in inevitably come from the system.  Solids capture and treatment always leave what can be a high strength liquid stream that can play havoc with your WWTW unless this side stream has been factored in.  

Contrary to all this technology should you have warm temperatures and endless sunshine then the cheap and simple solution is to use sludge drying beds. Dried sludge has the equivalent calorific value as brown coal so you could burn the dried sludge.  No idea what this smells like though!  You can also use dewatering tubes, where you put the liquid sludge in, the membrane allows water to escape and you are left with the solid residues.

As an alternative to all of the above our Sludge Wizard (SW) is a biological treatment system which uses worms to treat liquid sewage sludge.  Low capital and running costs, the SW treats the sludge effectively and efficiently.  The resulting effluent waste stream has  BOD levels similar to that of domestic sewage, so it does not impact on the WWTW unlike conventional technologies   The resulting solid by-product (worm casts) makes for an ideal alternative to chemical fertiliser and has higher N, P & K levels way ahead if you just used conventional sludge treatment technology.

As an aside sludge disposal implies that you are just getting rid of it at lowest cost with the sludge having no value.  Sludge recycling (to agriculture) is attributing a value which can deliver benefits.  In agricultural terms it is recycling nutrient N, P & K and organics which are vital to plant growth.