Understanding filtration › 5. Processes by sector

CHAPTER 5.5

Industrial sludges

Every water treatment plant produces sludge. In other sectors the filter cake is a product; here it is a waste that the site pays to have removed, by weight. Filtration is therefore judged first on the dry solids content of the cake, and on what it costs to achieve it.

Nicolas Ract · 8 min read · October 2026

Where separation sits in the process

General guidance: an industrial water treatment plant concentrates pollution in the form of sludge, which then has to be dewatered. There are two main families:

• physico-chemical sludges: the pH is adjusted, a coagulant and a flocculant are added, and the metals or suspended solids precipitate as hydroxides or flocs, which settle. This is typical of surface treatment, electroplating and metalworking shops;
• biological sludges: bacteria break down the organic pollution and multiply; the surplus is drawn off from the basin. This is the case in food processing, paper-making and organic chemistry.

The sludge treatment line generally follows the same sequence: thickening by settling or flotation, conditioning, mechanical dewatering, then removal of the filter cake, possibly after drying or liming.

Conditioning

Raw sludge consists of fine particles and bound water; it would blind the cloth within minutes. Conditioning gathers the particles into larger, more permeable flocs. For pressure filtration, a fact sheet from the US Environmental Protection Agency states that chemical conditioning is required, except for sludges very high in mineral matter [1].

• Inorganic conditioning: lime and ferric chloride, the traditional conditioning for filter presses, at 10 to 30 % lime and 5 to 7.5 % ferric chloride relative to dry solids, according to the same source. It gives cakes of more than 40 % dry solids and stabilises the cake, but the reagents can account for 15 to 40 % of the cake's dry solids, which reduces the weight saving [1].
• Polymer conditioning: it adds little mass, but is trickier on a filter press; cycles can exceed three hours [1]. It is the usual conditioning for belt filters, where the sludge-polymer mixing must remain gentle so as not to break the flocs [2].

Field experience: conditioning is the first variable to check when filtration deteriorates. A drifting dose, an aged polymer solution or a lime of different quality changes everything, and the cloth is often wrongly blamed.

The equipment

• The recessed chamber filter press: field experience, this is the most common equipment in industrial physico-chemical treatment plants. It produces dry, compact cakes: according to the source cited, pressure filtration takes many sludges above 30 % dry solids, in cycles of one to three hours with lime and ferric chloride conditioning [1] (see chapter 2.1).
• The membrane filter press: it generally gives drier, more uniform cakes, copes better with polymer and shortens cycles for a given dry solids content [1] (see chapter 2.2).
• The belt filter: continuous, with polymer, for biological sludges in particular. Typical dry solids contents vary widely with the sludge, for example 20 to 35 % for undigested waste activated sludge [2] (see chapter 2.3).
• General guidance: centrifuges and screw presses are also widely used; they do not use a cloth and fall outside the scope of this guide.

What wears or blinds the cloths

• Scaling: with lime conditioning, scale builds up in the cloth. The filter can be filled with an acid solution to dissolve it, but acid washing can shorten cloth life [1] (see chapter 4.4).
• Excess polymer: it forms a sticky film that clogs the cloth; field experience, this is a frequent cause of rapid blinding.
• Fats and oils, particularly in food-processing sludges.
• Washing: on a belt filter, high-pressure washing is recommended at least daily [2]; if poorly adjusted, it wears the cloth.
• Mechanical causes: on a belt filter, cloth replacement is the main maintenance expense, with an average life of around 2,700 operating hours and a very wide range, from 400 to 12,000 hours, according to the source cited [2]. For filter presses, see chapter 4.2.
• Field experience: in a physico-chemical plant, a change in the shop's baths or effluents (a new product, the dumping of a concentrated bath) changes the nature of the sludge overnight. The plant logbook should record it.

What matters to the operator

Dry solids content, regulatory or contractual

In France, the arrêté du 15 février 2016 relatif aux installations de stockage de déchets non dangereux (French ministerial order of 15 February 2016 on non-hazardous waste landfills) bans the acceptance of liquid waste and of waste with a dry solids content below 30 % [3]. For a site whose sludge goes down this route, 30 % dry solids is therefore a floor. The other routes, agricultural land spreading, incineration, co-incineration, hazardous waste landfill, have their own requirements, set by the applicable regulations or by the contract with the contractor. Field experience: reread the removal contract; it often sets a minimum dry solids content, a cake form (shovelable, no free water) and penalties.

Disposal cost

The filter cake is invoiced per wet tonne: transport, treatment and, where applicable, tax. Each percentage point of dry solids reduces the tonnage to be paid for, as shown by the formula "wet mass = dry mass ÷ dry solids content" in chapter 6.3. But conditioning reagents also add mass: the reasoning must be based on the sludge actually produced, reagents included [1].

The rest

• Filtrate quality, as the filtrate returns to the head of the plant.
• Operator time: preparing the cloths, cake discharge and washing can be labour-intensive on a filter press [1].
• Working conditions: when the filter is opened, operators may be exposed to odours, sulphur compounds and ammonia [1].

Indicators to track

The "Minerals or industrial sludges" case in chapter 4.5 provides the framework. In practice:

• Service life unit: tonnes of dry solids filtered, or cycles; operating hours on a belt filter.
• Per cycle or per day: cake dry solids content measured by the site, cycle time, filtrate appearance.
• Conditioning: reagent doses per tonne of dry solids. A dose that rises to maintain the same dry solids content is a warning sign.
• Washes: date, type (water, acid), effect on the following cycle time.
• Cost: wet tonnes removed and total cost per dry tonne, reagents and cloths included (see chapter 6.3).

Choosing the medium: points to watch

• Start from the conditioning: a cloth for limed sludge must withstand scaling and acid washes; a cloth for polymer-conditioned sludge must above all give good cake discharge and wash easily.
• Cake discharge: on a filter press, a sticking cake ties up an operator at every opening; a smooth surface, monofilament or calendered, helps (see chapter 3.2).
• Retention: enough for a clean filtrate, without blinding (see chapter 3.3).
• The fibre: compatible with the pH of the sludge and of the washes (see chapter 3.1).
• Judge on cost per tonne: the price of the cloths is small compared with the disposal cost; a cloth that gains dry solids or spaces out the washes pays for itself quickly, to be confirmed by a comparative trial (see chapter 6.2).