Design Of A Wastewater Treatment Plant: Sludge Thickener And Sludge Dewatering

Sludge Thickener

Thickening refers to the procedures that are used to increase the solid content of the sludge through the removal of the liquid fraction. The process of thickening is usually accomplished through physical means which include the co-settling, floatation, centrifugation, gravity settling, rotary drum and gravity belt (Ainsworth, 2016). The volume reduction which is obtained  by the sludge  concentration  is very useful  to the subsequent  treatments  process, such as  digestion, drying ,dewatering  and combustion from the following standpoints

1. Quantity of the chemical that are needed for the sludge conditioning
2. The capacity of the tank and the equipment that are required
3. The amount of heat needed by the digester and the amount of the auxiliary fuel need for the heat drying or the incineration

The table below gives the most common used methods of thickening in sludge process.

Table 1: Thickening methods used in the sludge process

The sludge thickener location within the   wastewater treatment plant is very important.  In the situation where sludge is intended to be thickened a mix of the waste activated sludge and primary sludge is an appropriate culture. If the resulting sludge’s are intended to be dewatered, they should be separately thickened and then blended instantly before the dewatering process is carried out.

Activities of a sludge thickener

 This sludge thickener usually operates with the same principle of the settling tank. The sludge enters the thickener from the middle and then it is distributed radially, they are then forced to settle and then collected from the underflow, the effluent usually exists over the weirs.

 In the thickness which operate for long. There are difference areas of concentration. In most cases the topmost zone is solid free and it usually comprises the liquids which are later removed over the weirs.

 In a thickener which is continuously operated, there are various zones of concentration. Usually the topmost   dear zone is free of the solids and it usually comprises the liquids that are eventually escaped over the weirs. The next zone is usually referred to as the feed zone .In this zone it is characterized by a uniform solid concentration .Below the feed zone is a zone of the increasing solid concentration. This zone is referred to as compaction zone.

The sludge  blanket  is defined as the top of the feed zone ,The  thickness of the blanket  is the key  operational control that  the treatment  plant operator has over  the thickener. Through the increase of the  underflow rate , the operator  can lower the  blanket , and hence  the solids residence  time is also lowered, throughput  of the solids  is increased  and the  solid concentration  in the underflow  is decreased. The operator then would have a higher reserve volume in case there is unexpected heavy sludge load coming. In the situations where the sludge blanket is high it will lead to the underflow solid concentration to be high because of the high solid residence time (Turovskiy, 2017). One challenge with this approach is the formation of gas due to the anaerobic activity. The gas formed will cause the floatation of the solids in the thickener. The chemicals such as  chlorine  need to  be added  in order to  inhibit  the biological  activity .A well  operated  thickener  will have a solids recovery of approximately 95%.

Design of thickeners

Thickening Methods Used in the Sludge Process

The thickening process usually takes place in the settling tank with a  long-enough solid retention  time. Actually the  thickening  o0f the  sludge  is of  great concern  to the operator where he desires  a high  underflow  solids  concentration. So  is the general  practice  to design  these process   in order to  achieve  better performance and clarification, There are two techniques of designing the thickeners  which include;

1. Design based on the experience
2. Design based on the laboratory  data

 Solids via output is very crucial criterion in the design of thickeners. The design is usually based on the solids flux. That is kg solids/h/m² .Typical flux values are illustrated below.

The design by the use of this approach  involves the selecting  of a typical  solid flux  and then calculating  the required  surface area through the division  with the anticipated  solid feed  by the  flux (Brandt, 2010).

Are of the thickener is usually obtained through the formula below

Design based on the laboratory data

This is the best method of design thickeners if the laboratory data is available. A typical test is usually carried out by using a 1000ml-graduated cylinder. Then the sludge is mixed homogenously and let to settle in the cylinder. In a few moments an interface which is aimed at separating the solids and the clear water at the top is carried with a given settling velocity (Association, 2013). This velocity of the interface is determined with respect to the time. Interface height is then plotted against the zone settling velocity which is calculated from the initial slope of the graph. The graph is shown in the figure below.

Fig 1: Interface during settling test.

SLUDGE DEWATERING

Sludge dewatering refers to the separation of liquids and solids whereby, generally, the least possible residue moisture is required in the solid phase and the lowest possible particles are needed in the separated liquid phase. A good example of this process is the dewatering of sludge from the municipal sewage plant or the treatment of industrial waste waters (Jeffrey J. Peirce, 2015).

The process of dewatering usually lowers the weight and volume of the solid wastes, thus reducing costs such as landfill and transport at the same time it enhances its suitability for the subsequent utilization. The process stabilizes wastes   while reducing leachates and offers a more uniform final product. The performance from the  mechanical  dewatering n is very critical  to  downstream, energy  recovery  from the combustion .While the polymer  use can improve  dewatering  the process is expensive and  can be challenging  downstream.

The option of mechanical dewatering  usually dewater sludge  to 30 to 50 percent  solid content  depending  on the combustion  and the ability to retain sludge .Nevertheless with the  paper and pulp  sludge  solid contents of 20-30 %  are mostly achieved.

The primary solids in the sludge are made up of fines, fibers and they are simple to mechanically   dewater. The secondary sludge from the biological and the chemical treatment ids very complex to dewater mechanically and the secondary solids are commonly combined together with the primary sludge to enhance its dewatering properties.

Chemical  additives such as  acids,  inorganic   conditioning  compounds and  surfactants  are applied  to enhance  flocculation  properties  and the dewatering. Thermal conditioning  of the biological b sludge by  heating  is believed to  enhance  dewatering  to 30-40 % b solids as  opposed  to the  up 20%  solids  for the chemical  conditioning (Moore, 2010).

Dewatering aids

The sludge is normally conditioned prior to its thickening and dewatering. In most cases two types of condition chemicals are applied to improve the dewatering of sludge and they are;

1. Mineral chemicals such as lime and iron salt. The chemicals are often found in the filter press applications
2. The organic chemicals such as coagulants and the flocculants .The most common type of the flocculants are usually encountered in nature.

Centrifuge dewatering

In this method the centrifuges dewaters continuously with the aid of centrifugal forces of many thousand gs.

The  principle of  a centrifuge is also referred to as  centrifugal  decantor , this principle is to use  the centrifugal  force in order to  accelerate  the solid-liquid  separation. For  the purpose of  simplifying  it can be assumed that  a centrifuge  is a conical  cylinder  decantor  which turns  horizontally  on its  own axis with a clarified  water flow , the dewatered  sludge which is being  removed  by  an Archimedean screw (Bajpai, 2014). The rotation applies a centrifugal force on the solid particles which then rotates at very high speeds.

In real situations the flocculated sludge is injected inside the centrifuge bowl via the injection pipe. The bowl usually rotates at very high speeds of approximately 350rmp and the particles are flattened against the walls of the bowl in the clarification zone. The particles are then moved  by the Archimedean  screw  towards the  end of the  bowl’s  cone  in the  sludge  spin-dry  zone .The clarified  liquid is referred to as  centrate  is  removed  at the  other end of the  bowl  by the overflow. The figure below show the centrifuge.

Fig 2 centrifuge.

References

Ainsworth, B. (2016). Handbook Biological Waste Water Treatment – Design and Optimisation of Activated Sludge Systems. London: Webshop Wastewater Handbook.

Association, A. W. (2013). Water Treatment. Sidney : American Water Works Association.

Bajpai, P. (2014). Management of Pulp and Paper Mill Waste. Berlin: Springer.

Brandt, M. J. (2010). Water Supply. Chicago: Malcolm J. Brand.

Gregorio, D. D. (2016). Chemical Primary Sludge Thickening and Dewatering, Volume 1. London: Municipal Environmental Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, .

Jeffrey J. Peirce. (2015). Environmental Engineering. Los Angeles : Butterworth-Heinemann.

Moore, B. A. (2010). Process control and troubleshooting in municipal sludge thickening. New York : University of Wisconsin–Madison,.

Turovskiy, I. S. (2017). Wastewater Sludge Processing. Texas: John Wiley & Sons.