Sludge dewatering: tackling costs and performance
Sludge dewatering is a critical process in the wastewater treatment process: reducing water content and weight (& volume) of sludge to facilitate easier handling and transportation and to enhance resource recovery. Increasing dry solids %DS content (thus removing excess water) significantly decreases sludge volume.
This often is a necessary requirement for further steps in sludge cake processing. Due to stricter targets for sustainable treatment plant operation and rising energy and chemical dosing costs, sludge dewatering optimization is a hot topic in the industry.
The cost of dewatering
The efficiency of sludge dewatering directly impacts operational costs and environmental performance of your WWTP. At the input-side sludge dewatering requires polyelektrolyte dosing and a significant amount of energy, both rather expensive. At the output-side, dewatering centrate or filtrate with low quality (such as a high content of suspended solids or unbound polyelektrolyte) poses a large burden on the wastewater treatment plant.
In order to fully optimize the sludge dewatering installation, online measurements are necessary. These measurements are applicable to decanter centrifuges, but as well to thickening tables or belt presses alike. Olpas' measurements focus on improving dewatering as well as reducing operation cost. The goal? A clean centrate & dry sludge cake at a minimal energy and chemical dosing requirement.
Our goal? A clean centrate & dry sludge cake at a minimal energy and chemical dosing requirement.
In order to answer one of the ‘Why?’s for optimization, consider the following graph, which shows the energy consumption and production in relation to the original moisture content.
Ignoring chemical and other operational costs, the sludge cake processing costs rapidly decrease with increasing dry solids content. Drier sludge is reduced in volume and weight, considerably reducing transport cost as well as further processing costs such as sludge cake combustion.
What does it take to optimize?
Sludge dewatering is defined by a stream of ingoing sludge at a certain concentration (often 0-100g/L) going towards the dewatering installation. There, the centrifuge or press separates the solid content from the liquid phase, often aided by the addition of polyelektrolyte emulsions. Discharge streams are the centrate or filtrate and the dewatered sludge or sludge cake. An efficient dewatering process is charaterized by the following parameters:
A known content of ingoing sludge (ton/hour) fed to the dewatering system
A low Total Suspended Solids (TSS) content in the centrate or filtrate
A low content of (residual) polyelektrolyte in the centrate or filtrate
High dry solids or dry matter content in the dewatered sludge
These parameters are the key defining parameters in the ingoing and outgoing streams of a dewatering plant. Online and reliable measurements of these values allows for implementation of real-time control systems.
At Olpas, we developed a set of instruments to do exactly that:
Ingoing sludge concentration sensor: Ultrasound backscattering for accurate solids loading on the system
Centrate residual measurement: online measurement of TSS and PE content in the centrate of filtrate, by Olpas Technology [patent pending]
Sludge cake dry solids content: Ultrasonic impedance measurements show the dry solids or moisture content remaining the sludge cake
OK: Let's measure and optimize!
Patience! It took us several years to develop this strategy and the accompanying products. We’ll need a few additional blogposts to dive deep into it!
So that will be the content of the following blog posts in this series.
We will explore several measurement and control strategies applicable to small scale (<20k P.E.) and large scale installations (>35k P.E). We will dive into some good practices for measurements and their respective benefits.
But already a spoiler: Any sensor installation with feedback on operational parameters is break-even from cost and sustainability point of view within several months!
We’ll build it up like this:
Post 1: Introduction (That's this post)
Post 2: Feed-forward control as optimization strategy: using the sludge inlet concentration measurement for precise control decanter loading and PE dosing.
Post 3: Centrate/filtrate residuals measurement as a feedback loop on polymer dosing and efficiency
Post 4: Why sludge cake dry solids content is invaluable for large scale dewatering installations
Post 5: Let’s recap: why optimization of your process is necessary. Cutting operational cost and running a plant efficiently.
With the final goal in mind: Dry sludge cake at a minimal operational cost and a sustainable process.
