Decision Criteria for Sludge Handling Devices

By Dr. Hans Valerius
March 2004

The Author is Director at Enigma Process Technologies Corp in Muntinlupa, Philippines. His company provides engineered solutions for laundry waste, hospital waste, animal blood, animal waste, palm oil mill waste etc. → See also:

As a matter of fact in sludge treatment various choices are given which after careful consideration are limited to finally three possibilities of adequate sludge treatment devices.

Those who are confronted with the decision taking, dealing with a given budget do go through a difficult process to decide which of all the opportunities offered by the representatives of filtration and separation device manufacturers.

Those investment costs that are within the allocated budget are the most preferred ones and the standard question by the sales engineer, how much is in your budget is often enough a guideline to submit a proposal which makes any decision by the purchaser easier. This attitude however is wrong and in many cases causes the operation of any filtration and separation system in a company unforeseen running costs which are far beyond any expectation. Running costs in several aspects, the replacement costs, the operation costs, the number of operators involved, the excessive sometimes needless use of filter aid etc.

Being involved in sales we are applying proper business ethics of course not telling lies while presenting our proposal but we are also not telling exactly the truth. All our indications, technical specifications are in conformity with what we intend to supply to the end user, finally we are selling a philosophy and philosophies are always discussable.

The end user himself cannot know all the pros and cons of the individual systems that can be applied for his sludge treatment. Asking for warranty and guarantee is not always in his mind and consequently a decision is taken which is at convenience within the planned budget. The eye opening fact that may be company´s money was wasted comes naturally at a later stage. This can be after 2 years, latest after 5 years the picture is clear.

We are referring here to the most suitable sludge treatment devices such as filter press, belt press and decanters.

Investment costs for a belt press can be lower than for a filter press and a filter press again can be lower than a decanter, all subject to the quantity of the sludge to be treated and its parameters in terms of solids, expected dry solids etc.

But there is a long list of other costs involved. Replacement of parts, wear and tear parts, filter media, filter aid, number of operators, energy consumption, space requirement, continuous or discontinuous system and many more.

To compare the most common three systems filter press, belt press and decanter we can summarize as follows:

  Decanter Centrifuge Belt Press Filter Press
Installation $$ $$$ $$$$$$
Building Space + ++ +++
Maintenance + ++ +++++
Solids Disposal $ $$ $$
Ventilation * *** **
Water + ++++++++ ++++
Labor + ++ ++++
Energy ++ + +
Closed System YES NO NO
Continuous System YES YES NO
Labor intensive NO YES YES

A sales engineer selling belt presses might defend the right of existence of his belt press but cannot deny the above facts. Continuous system, yes, for sure but without having an operator there to check from time to time on the performance of the belt press can result in:

sludge is exposed to air

The sludge is exposed to air and can cause unpleasant smell which is not a technical problem but simply a normal appearance with belt presses. A damaged cloth requires hours to be replaced not to mention the costs. Uneven cake, required sludge thickener and the high consumption of polymer can cause a figure in the annual operation costs which after a certain period of time exceed by far the original investment costs.

No doubt filter presses which are as a "cons" argument working as a batch system and which require at least 1 operator if not 2 deliver a good cake all provided the many steps in the operation and in the outfit of a filter press were considered.

sludge is exposed to air

Although appearances like the following are also possible as a rule of thumb the system works quite nicely if there would not be the costs involved which makes it an item in a production or in a sludge treatment which can turn out to be a real cost consumer.

Looking at the price level we concluded that a belt press is lower in price than a filter press. Furthermore we concluded that a decanter is higher in investment costs than a filter press as a general statement. But how is the reality of comparison. If we forget again the consumption of polymer which can be high with a filter press operation, the bigger space requirement, operators, batch system and replacement costs of filter cloth and filter elements we can summarize some of the hidden costs as follows:

To compare a filter press with the operation of a decanter on a real fact finding mission the comparison should look as follows:

For the filter press For the decanter
Plate shifter No requirement is already automatic
Plate shaking device No requirement no need for such a device
Plate washing system No requirement
Splash curtains, Bombay doors etc No requirement
Filter elements No requirement
Filter aid Subject to separation task mostly lower consumption
Air/Water squeezing system in case of membranes No requirement

Well, a plate washing device is surely a technical solution however how are the dry solids in the final stage after cake discharge followed by plate washing in spite of Bombay doors being installed. Theoretically and assuming the average operation procedures we add liquid into the dry solids although in small amounts always provided proper operation of the filter press is done.

The fact that in case of a filter press equipped with membrane filter elements to achieve higher dry solids compared with standard recessed plates represents about 50 % of the investment costs of the filter press is food to think about, bearing in mind that these membranes need to be replaced and do not last for ever but require a replacement as an average after 4-5 years if not earlier. Not properly cleaned channels of the filter elements, a decanter does not have such a requirement, can lead to early filter element damage and with filter elements which are mostly having weak hinge areas as seen below

damaged filter elements

the basic outfit of a filter press to keep it operational can turn out to cost double of what has been invested before. Over a period of 10 years even more than that not considering the replacement of filter cloth, leaving aside that in many cases calendered filter cloth for better cake discharge is required. The scrapper to scrap off remaining dry solids still adhering on the filter elements after opening the filter press should be available at any time of the operation.

Why filter elements are an expensive item cannot be answered considering an average price of polypropylene of 1-1.5 US Dollar per kg but turning out into a price of roughly 2,000 US Dollars and more for a bigger sized membrane with about 80 kg material weight. Attractive alternatives of detachable membranes where only the damaged part of a membrane has to be replaced is another cause of headache with the operation and maintenance costs since such plates are basically not much different from the production costs of a one-piece membrane element.

Going through the experience with a filter press the end user will sooner or later find out that the investment was based on false assumption and that the attractive low initial costs are actually an installment payment for a system which compared on technical grounds and performance and matching comparison taking all the features of a decanter into consideration will not have the initial attraction anymore.

Among decanters as well as among other filtration and separation devices there are of course various systems and again competition while the sales engineers will try hard to book an order from the end user. Again we reach the border of telling the real truth behind each system. Polymer consumption with a decanter application can be low subject to the retention time that has been selected. Within the various models there are deviations in length, G Force, angle of discharge or in proper terminology "dry beach". A decanter with a higher angle can provide a serious problem when it comes to the desired separation result. A missing built in scraper can cause additional costs during operation when the solids left in the decanter are hardening and can only be removed by opening the decanter. Required man power, pulley and not to mention the interruption in the production are matters to be worth being considered before any decision is taken.

After all comparing the three most common systems in sludge treatment a filter press might be a solution for very small quantities where all the automatic functions and created accessories to make the operation which still remains a batch system are not required. Considering the sensitive belt press a clear no should be the answer whereby belt presses to perform excellently in other industries where the alternatives filter press and decanter for reasons of solids to be handled cannot or should not be used, however, again this area is very limited.


Among the decanters the end user should focus on the existence of a built in scraper, an adjustable feed pipe, adjustable while the decanter is in operation of course since otherwise an extra work load and interruption will be the case and finally not to forget that a decanter is equipped with studs allowing sufficient space beneath for the solids discharge.

Overall results can be shown as follows:

As it is with investments and decision taking budgets are prevailing and the decision maker himself cannot and does not have the technical background knowledge to decide which system is the better choice. These areas of filtration and separation devices cover complex topics which are not known to the end user. To avoid wrong decisions or better to be protected against enormous operation and maintenance costs the end user is well advised to ask any supplier of such systems for a break down of costing during operation insisting on warranties and guarantees. Only then the scenario will be a different one since the approach of any sales engineer in promoting his product will be stripped down to facts and figures rather than theoretical statements.


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