How does your sludge flow?

Sludge-mixing is regarded by many as a black art. Dr Mick Dawson (BHR Group) illustrates how mixing processes can be quantified and how cost savings can be made

There are a great many sludge-mixing applications. For instance, sludges from different sources with different physical characteristics can be blended together in tanks or in-line to provide more uniform and consistent feed for downstream processing. Balancing and storage tanks are required upstream and downstream of sludge processes to enable operation at constant flow rate. These tanks need to be effectively blended. Poor sludge storage or balancing tank mixing will result in problems with process control, inadequate treatment stability and higher revenue costs.

Anaerobic digester mixing is vital for the blending of digester feed sludge, for making use of the whole digester volume, diluting inhibitory substances, avoiding temperature or concentration stratification and maximising gas production. The increased emphasis on pathogen destruction in digesters has dictated retention times in the digester must be optimised by avoiding dead-zones and short-circuiting. There is also a need to digest thicker sludges in order to increase throughput.

Then there are sludge-thickening and dewatering processes, the effective operation of which is dependent on the upstream addition of sludge-conditioning chemicals such as polyelectrolytes. The rate of mixing of the polyelectrolyte can have a big influence on the thickening or dewatering performance as well as the dose of chemical needed. Getting the polyelectrolyte mixing right can therefore improve dewatered sludge dry solids, hence reducing the cost of transport and/or further processing. For example, on one belt-press plant, intensifying the polyelectrolyte mixing improved polymer effectiveness (kg dry solids/kg polymer) by 40%.

In addition to the common sludge-mixing applications described above, there are plenty of novel processes being adopted by the industry that require mixing steps, such as thermal and enzymic hydrolysis, various chemical treatments and pasteurisation. Some of these novel processes involve sludges with distinctive properties that have not been encountered before, making the task of mixer design or selection very difficult.

It is in the mix

In order to get the best from a process, we must quantify the mixing performance and compare that with the mixing performance we think is needed. To do this we can look at the flowpattern, the residence time and the mixing rate.

The flowpattern describes the way sludge flows through the tank or mixer; the two ideal (extreme) flowpatterns are plug-flow and fully back-mixed. Most processes should operate at one of these ideals but in practice non-ideal short-circuiting, stagnant regions or recirculation loops are present. The flowpattern can be quantified by measuring or modelling the distribution of sludge age as it flows through the tank or mixer (Residence Time Distribution).

Most sludge-mixing applications involve blending or homogenisation. To quantify this we need to determine mixture quality (concentration variation) and time taken to reach it (blend time or mixing time). Mixture quality is described using the Coefficient of Variation (CoV) which is a statistical measure of concentration variability. The smaller the value of CoV, the better the mixture quality. A CoV of 0.05 is a frequently used target value – this roughly equates to measured concentration varying within +/-10% of the mean value. Blend time or mixing time is the time taken to reach a specified mixture quality.

Quality and quantity

When it comes to the specification and design of sludge mixers, there are two key questions that need answering. How good does the mixing need to be to optimise the process? And how can you achieve that degree of mixing?

The answer to the first question requires consideration of the mixing objectives. This should always be carried out before mixing equipment is sized or selected. Digester mixing systems, for example, might aim to achieve an actively mixed sludge volume of 90%, ensure that the short circuit volume is less than 5%, and blend feed sludge into the bulk within two hours. Polyelectrolyte conditioners dosed into sludge prior to dewatering may need to be blended to CoV 0.05 within one or two seconds. This rapid mix should be followed by a period of less intensive shear for floc conditioning. Sludge storage tanks may need to be mixed to CoV 0.05 prior to emptying.

It is important to realise that the optimum degree of mixing (especially of dosed chemicals) may be specific to your process and hence may need to be determined by jar tests or similar scale-down procedures.

Counting the cost

Once quantified mixing objectives have been decided upon and other constraints identified, the most cost-effective mixing equipment selection can be made. Not all mixer types/designs require the same amount of energy to achieve a specific mixing objective, so it pays to understand the capabilities and relative performances of the available kit.

There is a large selection of equipment available for sludge-mixing. In-tank or digester systems include numerous types of gas injection, pumped liquid jets, top-mounted impellers and submersible mixers. There can be major differences in energy consumption for these systems. Investigations undertaken at BHR Group has shown that well-designed sequential gas-mixing systems can significantly outperform those where gas is supplied continuously to multiple nozzles. For example, replacement of continuous mixing systems with sequential ones realised a power saving of £4,000/pa per digester at one site in southern England.

In-line sludge and conditioner mixing can be achieved using T-mixers, static mixers, valves, pumps and dynamic (rotating) mixers. The mixing efficiency and performance of these devices varies dramatically, as does their tendency to block. However, little or no published information on the relative performances of this important class of equipment is available.

Go with the flow

It is vital to know how your sludge flows (sludge rheology) when selecting a mixer for sludge applications. Sludges often exhibit non-Newtonian shear-thinning rheology where viscosity varies with shear rate and hence position within the tank or mixer. In addition, some sludges exhibit a yield stress which must be exceeded if the sludge is to flow at all. Sludge rheology is difficult to predict on the basis of dry solids, temperature and sludge type alone; this is particularly so for sludges associated with newer processes. Sludge rheology should therefore be measured to enable sludge process equipment to be designed properly.

The performance of in-line sludge mixers is drastically influenced by whether the prevailing flow regime is laminar or turbulent (turbulent mixing being much faster). The flow regime is in turn strongly influenced by sludge rheology. Without an accurate knowledge of the rheology of your sludge it is difficult to determine mixing performance accurately and realise potential savings.

Mixing performance can be measured using physical or computational model studies as well as on-site tracer testing. Mixing research for the chemical process and water industries has relied on model studies at different scales to identify the influence of variables and to build up predictive correlations for mixing performance. On-site tests can be used as a diagnostic tool and to validate the performance of installed equipment.

Cutting costs

Attention to sludge-mixing pays. The first step is to define the mixing objectives for your sludge process application. Next it is important to measure the flow behaviour of your sludge. You can then select the most efficient mixers to achieve your mixing objectives. Ideally this should be based on independent information; alternatively, mixer suppliers can be consulted. Once the chosen mixers are installed, the mixing performance relative to your objectives should be measured on site to confirm the suitability of the installed equipment.

There is considerable scope for reduction in sludge-processing costs through improved understanding of the link between mixing and process performance. Better knowledge of relative mixer performance will lead to optimised processes and the emergence of more suitable mixer designs.

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