help in analysing malfunctions

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Wastewater treatment plants, in particular those using activated sludge processes, may be subject to failures which limit the reliability in the performance of purification operations.
These problems appeared very early during the development of activated sludge processes but their frequency increased when processes designed to removal nitrogen and phosphorus were introduced.

Several factors are likely to affect the behaviour of activated sludge and include:

  • the type of wastewater and network
  • plant design
  • plant operation

Some of the most common problems encountered include:

  • non-compliance to standards
  • foaming
  • profuse filamentous development (sludge bulking)

the type of wastewater and network

Industrial discharge must be controlled and the discharge of FOGs, brine, heavy metals, toxic substances (phenols, cyanide, etc.) into the networks must be banned.

In long networks with shallow slopes water is often in a state of septicity. To avoid fermentation a velocity higher than 1m/s must be ensured.

In long networks, treatment stations can be included (injection of oxidants, metallic salts, etc.).

The sulphur concentration level in incoming effluent must be lower than 1-2 mg/L. Regular network cleaning guarantees good working order.

plant design

  • The pumping station: pump located at the lowest point, slopes for encouraging the transfer of deposits to the pumping area, good quality civil engineering, etc.
  • Storm water and buffer tanks: provide for fast return and include an agitation system to avoid deposits
  • pretreatment: realistic speed in the grit remover and calm zone at the grease trap outlet in order to improve the concentration and take-up of fats, oil and grease

primary settling

To limit retention time to 2 hours, the tank must not be over-sized. Sludge must be extracted as regularly as possible and at concentration levels lower than 10g/L. Primary sludge must not be thickened in the primary settling tank. A specialised unit must be included (e.g. static thickener).

biological tanks

  • Contact zone
  • If there is only one biological tank (ditch)
  • Anoxia, anaerobic: Avoid a design where floating material will accumulate (overflow outlet) and ensure good uniform mixing.
  • Aeration tank: Preferably use overflow outlets in order to avoid the storage of floating material. The aeration unit must be sized correctly (compressor and distribution of aerators) in order that the power is sufficient enough to avoid deposits and dead zones.
  • Degassing: Include a degassing tank prior to the clarification process especially when the height of the liquid in the aeration tank is higher than that of the clarifier in order to remove dissolved gases (nitrogen and air) and if the height of the drop is higher than 50cm.
  • Clarifier

This must be sized according to the flow rate, the sludge's concentration level and how easily the sludge can be settled. Sludge uptake (scraped or suction tubes) must be uniform along the entire floor of the tank. A recycling rate of 150% must be envisaged in order to avoid storing sludge for too long especially during peak flow rates.

plant operation

non compliance to standards

Regardless of the nature of the problems related to water and to the facility's design, good plant management is essential if standards are to be adhered to. To avoid parameters deviating from the normal values it will be necessary to adjust operating parameters:

  • the inlet biological load
  • the characteristics of the inlet water
  • the temperature (minimum age of sludge for nitrification purposes)
  • correct oxygen supply
  • mixing must be ensured (to avoid deposits)
  • regularly extract excess sludge (to maintain aerated sludge whose age is close to the minimum age).
  • do not store floating material, essentially in non-aerated zones.
  • correctly recirculate sludge from the clarifier (limit sludge retention time to 2 hours).

sludge treatment

  • Make sludge removal operations more reliable in order to regularly extract sludge
  • do not send material resulting from digestion back to the head of the process, good operation of digestion process with low concentration levels of volatile fatty acids ( VFA )
  • prohibit the return of pollution loads undergoing anaerobic digestion (thickening, dewatering, etc.).


The main malfunctions related to filamentous organisms in a treatment plant occur in two ways:

  • profuse filamentous development (sludge bulking)
  • foaming

The causes are often multiple and the choice of solutions requires a global analysis (see previous paragraphs)

profuse flamentous development (sludge bulking)

Profuse filamentous development or "sludge bulking", is the presence of filamentous organisms in flocculent particles (or between pieces of flocculent particles) which disrupt the sludge's settling properties (high sludge index, higher than 200ml/g) thereby compromising the quality of purified water. The nuisances caused by the filaments are proportional to their density and their length in particular. These also depend on the filamentous species present, with some being more harmful than others.

The studies carried out by different researchers, particularly Eikelboom and Jenkins, have resulted in the classification of filaments into 29 types. Table 8 in biomass used in wastewater purification shows the most frequently found types of filaments as well as the main causes of their development.

Taking into account the diversity of filaments and the conditions which may encourage them, the practical approach in the case of their profuse development consists in:

  • evaluating the extent of colonisation by filamentous micro-organisms.
  • identifying the filamentous species involved (examination under a microscope).
  • determining the appropriate remedies.

* in the short-term, treat the symptoms: modify the sludge recirculation rate, add measures to aid settling, chlorinate the sludge (see biomass used in wastewater purification for more details on the methods enabling the measures to be implemented).

* in the long-term, treat causes: check the septic nature of the incoming water, the nutrient supply, that there are no deposits, modify the mass load, aeration rate, etc.

A method often used is the chlorination of activated sludge, the principle of which is to destroy the filamentous organisms on the surface of the flocculent particles without affecting the flocculent's bacteria.

If treatment is to be effective, the strict conditions of application need to be adhered to:

  • appropriate amounts, from 3 to 8 kg of Cl2 per tonne of TSS present in the system and per day
  • injection point (recycling) excellent mixing, concentrated sludge, no competition with BOD5, NH4, etc.
  • suitable renewal rate, minimum twice per day
  • sufficient duration (from 2 days to 1 week according to amounts injected)
  • frequent monitoring of treated water quality.

Other oxidants, such as ozone or hydrogen peroxide, can also be used.


Foaming of activated sludge is a phenomenon which occurs when foam forms at the surface of an aeration tank and is either whitish in colour or brown (chocolate mousse).

There are 4 types of foam:

  • Start-up foam (light, whitish foam which disappear within a few days)
  • Surfactant foam (detergents, industrial input, colloidal organic matter (blood), hydrocarbons, etc.). If supply is regular, the phenomenon is limited in time. It it lasts, it may ultimately provoke the emergence of stable biological foam.
  • Unstable floating matter (origin: grease removal, degassing, denitrification, anaerobic digestion). This is characterised by the absence of filamentous organisms.Simply spraying reduces this. To avoid the emergence of a stable form, it is essential to remove floating material.
  • Biological foam

biological foam

This type of foam forms heaps of stable, brown to light brown coloured floating material which progressively covers the surface of biological tanks and may be transferred to the clarifier and then removed along with the effluent.

The two main filamentous organisms responsible for foaming which can be identified under a microscope, are Nocardia spp and Microthrix parvicella. These slow-growing organisms are commonly associated with septic conditions, deposits, non uniform conditions, grease, a relatively high temperature (higher than 18°C) and quite old sludge (more than 5 days old).

However the selective accumulation of Nocardia spp in the foam trapped at the surface leads to an increase in this organism's retention time and encourages its proliferation even in younger sludge systems.

Foaming due to Nocardia appears to involve the hydrophobic nature of the cell membrane which tends to cause floating at the surface of aeration tanks.

Due to its stability, this foam is very difficult to remove using chemical methods, even if spraying the sludge with chlorinated water can, in the short-term, constitute an interesting method.

The most effective methods against this must focus on the removal of the conditions which encourage the growth of filaments, i.e.:

  • removal of septicity ( VFA ), deposits, non uniformity, grease, etc.
  • reduce the age of sludge as much as possible
  • build tanks which do not allow floating material to accumulate: avoid submerged walls which trap the foam, build degassing wells which allow floating material to be removed.
  • remove floating material from biological reactors to avoid it from being recycled and avoid permanent reseeding.

resurfacing of sludge

A problem sometimes encountered with sludge with good settling properties is the return to the surface of flocculent particles or floating flocculent particles in the clarifier.

The two most common causes for this is degassing and denitrification. The problems are compounded by the present of filamentous organisms but not only

In both these cases nitrogen production (degassing of the over-saturated liquor and/or the transformation of nitrates into gaseous nitrogen) occurs through the formation of micro bubbles which attach themselves to the flocculent particles and cause floatation.

To resolve these problems implement the following measures:

  • Systematically include a degassing unit upstream to the clarifier and reduce the fall to 50 cm (degassing-clarifier)
  • Design the clarifier so that there is a possibility of recycling at a rate of 150%.
  • To avoid high temperatures, avoid operating with sludge that is too much old and/or high sludge concentration levels.
  • Optimize denitrification

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