As promised in the last blog the subject that is being discussed this week is sludge digestion and some of the more simplified control systems and instrumentation surrounding this area and the possibilities that exist.
Sludge digestion is probably one of the most common ways of dealing with sludge, getting a quality product from waste sludge and if you are really lucky recovering energy at the same time. Various alternatives exist such as incineration but sludge digestion whether it is mesophillic or thermophillic is something that, at least in the medium term, is here to stay.
The digestion process is also the one process within wastewater treatment that really lends itself to a subject that has been discussed on the WIPAC discussion group a lot in the recent weeks and that is HACCP.
In digestion there are a few variables that are critical to the process and as long as these are controlled properly then the process will work. For the sake of ease in this blog I will discuss single stage mesophillic anaerobic digestion (MAD) but the principles work equally for sites that have pasteurisation and or thermophilic Digestion (TAD).
So what are the control points and how does instrumentation and automation really fit in?
If we examine the products that digestion produces and work back from there things actually quite simple for quite a complex process, the outputs of the process are digested sludge that is largely pathogen free and biogas. If we take then products and see what the crucial points are we get something like diagram 1
In diagram 1 there is a simplified critical control point with a number of control points which if breached will affect the quality of the system. The first is digester temperature, MAD operates at 37°C with a range from 32 to approximately 39°C, movement outside this affects the production of biogas and if the temperature is too low the pathogen removal. The obvious instrument and control system here is the simple temperature monitor and control of the sludge re-circulation system. Temperatures approaching the lower end drive the boiler to put more heat into the system and the opposite for too hot a temperature. This is a very simple control system that already exists. Triple validation of the temperature probes within the digester ensures that the correct temperature is maintained.
The second control point is actually how the digester is fed, more typically this is based upon digested sludge volume, the tendency on many sludge treatment facilities is to push as much sludge through the digesters as possible. This decreases digester retention time to the point where pathogen reduction is compromised and the volatile fatty acid concentration rises to a point where gas quality goes down but this is actually related to the mass of sludge rather than the volume. The critical control point is of course sludge mass loading rates and ensuring that this is kept as consistent as possible. The type of sludge is also critical as well and this feeds right the way back to the wastewater treatment process and knowing what exactly you have in your treatment works as possible. What do I mean here? Well if as suggested in earlier blogs on a particular treatment works with sludge treatment facility the primary de-sludge mass is known along with the secondary sludge mass and of course any imported sludge then the ratio of primary to secondary to import sludge can be controlled. If these ratios are known then the amount of solids loading of the different types of sludge are known. With the information of the calorific values of the different types of sludge then the biogas production and quality can be predicted thus allowing a further critical control point being developed from the others on the basis of predicted gas production based upon the solids load into the process.
To monitor the predicted energy recovery on the basis of the calorific value of the different proportions of sludge put into the system although difficult is not an impossible task by today’s standards and when the amount of energy that is produced is based upon the performance of the digester(s) and the current value of carbon it can be seen that HACCP and automatic control of the digestion process is more and more critical moving forward.