Sludge Processing in Wastewater Treatment Plant

  1. Wastewater sludge

When wastewater is treated using various mechanical, biological, and physiochemical methods to remove organic and inorganic pollutants to levels required by the permitting authority, the sludge produced will also vary in quantity and characteristics from one treatment plant to another.

1.1Types of sludge

    Types of sludge and other solids, such as screenings, grit, and scum, in a wastewater treatment plant vary according to the type of plant and its method of operation. The sources and types of solids generated in a treatment plant with primary, biological, and chemical treatment facilities are illustrated in Figure 1.

        Wastewater sludge can be classified generally as primary, secondary (also called biological), and chemical. Sludge contains settleable solids such as (depending on the source) fecal material, fibers, silt, food wastes, biological flocs, organic chemical compounds, and inorganics, including heavy metals and trace minerals. The sludge is raw sludge when it is not treated biologically or chemically for volatile solids or pathogen reduction. When the sludge is treated, the resulting biosolids can be classified by the treatment, such as aerobically digested (mesophilic and thermophilic), anaerobically digested (mesophilic and thermophilic), alkaline stabilized, composted, and thermally dried. The treated sludge can be only primary, secondary, or chemical, or a mixture of any two or three of the sludge’s.

1.1.1 Primary Sludge

Most wastewater treatment plants use the physical process of primary settling to remove settleable solids from raw wastewater. In a typical plant with primary settling and a conventional activated sludge secondary treatment process, the dry weight of the primary sludge solids is about 50% of that for the total sludge solids. The total solids concentration in raw primary sludge can vary between 2 and 7%. Compared to biological and chemical sludges, primary sludge can be dewatered rapidly because it is comprised of discrete particles and debris and will produce a drier cake and give better solids capture with low conditioning requirements. However, primary sludge is highly putrescible and generates an unpleasant odor if it is stored without treatment.

1.1.2 Secondary Sludge

Secondary sludge, also known as biological sludge, is produced by biological treatment processes such as activated sludge, membrane bioreactors, trickling filters, and rotating biological contactors. Plants with primary settling normally produce a fairly pure biological sludge as a result of the bacteria consuming the soluble and insoluble organics in secondary treatment system. The sludge will also contain those solids that were not readily removed by primary clarification. Secondary sludge generated in plants that lack primary settling may contain debris such as grit and fibers. Activated sludge and trickling filter sludge generally contain solids concentrations of 0.4 to 1.5% and 1 to 4%, respectively, in dry solids weight. Biological sludge is more difficult to dewater than primary sludge because of the light biological flocs inherent in biological sludge.

1.1.3 Chemical Sludge

Chemicals are used widely in wastewater treatment, especially in industrial wastewater treatment, to precipitate and remove hard-to-remove substances, and in some instances, to improve suspended solids removal. In all such instances, chemical sludges are formed. A typical use in removing a substance from wastewater is the chemical precipitation of phosphorus. The chemicals used for phosphorus removal include lime, alum, and “pickle liquors” such as ferrous chloride, ferric chloride, ferrous sulfate, and ferric sulfate. Some treatment plants add the chemicals to the biological process; thus, chemical precipitates are mixed with the biological sludge. Most plants apply chemicals to secondary effluent and use tertiary clarifiers or tertiary filters to remove the chemical precipitates. Some chemicals can create unwanted side effects, such as depression of pH and alkalinity of the wastewater, which may require the addition of alkaline chemicals to adjust these parameters.

1.1.4 Other Wastewater Residuals

In addition to sludge, three other residuals are removed in wastewater treatment process: screenings, grit, and scum. Although their quantities are significantly less than those of sludge in volume and weight, their removal and disposal are very important.

Screenings include relatively large debris, such as rags, plastics, cans, leaves, and similar items that are typically removed by bar screens. Quantities of screenings vary from 4 to 40 mL/m3 (0.5 to 5 ft3/MG) of wastewater. The higher quantities are attributable to wastes from correctional institutions, restaurants, and some food-processing industries. Screenings are normally hauled to a landfill. Some treatment plants return the screenings to the liquid stream after marcerating or comminuting. This is not recommended because many of the downstream pieces of equipment, such as mixers, air diffusers, and electronic probes, are subject to fouling from reconstituted rags and strings.

Grit consists of heavy and coarse materials, such as sand, cinders, and similar inorganic matter. It also contains organic materials, such as corn, seeds, and coffee grinds. If not removed from wastewater, grit can wear out pump impellers and piping. Grit is typically removed in grit chambers. In some treatment plants, grit is settled in primary clarifiers along with primary sludge and then separated from the sludge in vortex-type grit separators. The volume of grit removed varies from 4 to 200 mL/m3 (0.5 to 27 ft3/MG) of wastewater. The higher quantities are typical of municipalities with combined sewer systems and sewers that contribute excessive infiltration and inflow.

Grit is almost always landfilled.

Scum is the product that is skimmed from clarifiers. Primary scum consists of fats, oils, grease, and floating debris such as plastic and rubber products.

It can build up in piping, thereby restricting flow and increasing pumping costs, and can foul probes, flow elements, and other instruments in the waste stream. Secondary scum tends to be mostly floating activated sludge or biofilm, depending on the type of secondary treatment used. The quantity and moisture content of scum typically are not measured. It may be disposed of by pumping to sludge digesters, concentrating, and then incinerating with other residuals, or drying and then landfilling.

  1. Sludge Processing

The purpose of primary and secondary treatment is to remove as much organic solids from the liquid as possible while concentrating solids in a much smaller volume for ease of handling and disposal. Primary sludge has a typical solids content of 4 – 6%. Sludge processing reduces the solids content of this sludge through biological processes and removes more of the liquid content of it prior to disposal.

The overall sludge processing investment cost at the typical wastewater treatment plant is about one-third of the total investment in the treatment plant. However, based on the individual wastewater treatment plant’s processing system, operating expenses in sludge processing typically amount to even a larger portion of the total plant operating costs. To reduce plant operating costs, it is essential to have a properly designed and efficiently operated sludge processing stage.

The design options for each process will be dependent on the type, size, and location of the wastewater treatment plant, and the solid disposal options available. The design must be able to handle the amount of sludge produced and converted economically to a product that is environmentally acceptable for disposal.

As with water processing, sludge process methods will be determined by the specific constraints and requirements of the individual wastewater treatment plant. Our schematic

covers the general processing steps found in a typical plant.

There will be many plant-to-plant variations that are not illustrated in our article highlights the processes of a typical wastewater treatment plant. The bottom half illustrates the Sludge Processing flowchart.

The typical sludge processing steps include:

  • Sludge Thickening
  • Sludge Conditioning
  • Dewatering
  • Disposal


Sludge Thickening

To optimize the sludge conditioning stage, it is important to maximize the solids content of the materials decanted from the water processing stages. The waste activated sludge, scum, and primary sludge can be thickened to reduce the liquid content prior to sludge conditioning. Due to the varying physical nature and liquid content of these materials, facilities may use different thickening processes and equipment for these three materials. In some cases, the primary sludge may not even be thickened and will be pumped directly to

sludge conditioning.

The intent is to optimize the downstream processing capabilities.

The four most common thickening methods include gravity settling, gravity belt thickening, dissolved air flotation, and centrifuge thickening.

The recovered liquid or supernatant from thickening is pumped back into the aeration tank or to the beginning of the water processing stage and is reprocessed.

Sludge Conditioning

Sludge conditioning is a key stage in the reduction of solids prior to disposal. Based on the size and location of the facility five common methods are typically utilized; chemical

treatment, anaerobic digestion stabilization, aerobic digestion stabilization, lagoon storage, and heat treatment.

Many facilities will have some type of aerobic or anaerobic digestion stage prior to dewatering. The purpose of sludge digestion is to convert bulky odorous sludge into a relatively inert material that can be rapidly dewatered without obnoxious odors.

Thickened waste activated sludge, scum, and primary sludge are pumped into the digester. In anaerobic digestion, the digester uses the naturally occurring anaerobic microorganisms

to break down organic materials into methane and carbon dioxide gases. The sludge is heated to 37°C (100°F) and agitated continuously in the digester to improve the rate of digestion.

There are two different anaerobic processes, single stage and two-stage. Single stage digesters utilize one digester (tank) to digest the sludge, capture methane gas and store the sludge until it is transferred to the dewatering process

Two-stage anaerobic digestion uses a primary and secondary digester. The primary digester is heated and utilizes mixers to completely agitate the sludge, which maximizes sludge digestion. The secondary digester is not agitated and is utilized for gravity thickening and storage of the digested sludge. The secondary digester typically incorporates a floating

gas dome for methane gas collection and supernatant is pumped out to increase solids content.

Anaerobic digestion is a biological process that breaks down a significant amount of organic solids in the sludge and produces methane gas that is utilized as a fuel for the plant.

Consequently, the volume of final sludge is greatly reduced, which in turn reduces the cost for sludge disposal.

The process also reduces the level of pathogenic microorganisms enabling digested sludge to be classified as biosolids that can be utilized as a soil conditioner or fertilizer.

Sludge can also be stabilized by long-term aeration that biologically destroys volatile solids. An aerobic digester is normally operated by continuously feeding raw sludge with intermittent supernatant and digested sludge withdrawals.

The digested sludge is continuously aerated during filling and for the specified digestion period after the tank is full.

Aeration is then discontinued to allow the stabilized solids to settle by gravity. Supernatant is decanted and returned to the head of the treatment plant, and a portion of the gravity thickened sludge is removed for dewatering.

The next step for the stabilized sludge is dewatering.



Dewatering is the final stage prior to sludge disposal. The goal is to economically remove as much liquid as possible from the sludge or digested sludge prior to disposal. The most common method of dewatering utilizes a belt filter press. The belt filter press has two continuous porous belts that pass over a series of rollers to squeeze water out of the sludge that is compressed between the two belts. Polymers are typically added to the process to enhance dewatering capabilities. Centrifuges are also used for dewatering, typically, in larger wastewater treatment plants.

Any supernatant that is removed in the dewatering process is returned to the beginning of the treatment plant for reprocessing.


Digested sludge that is processed into biosolids can be used to spread on farmland as a soil conditioner or can be further processed as fertilizer. It can also be disposed as landfill.

Sludge can also be incinerated and the remaining ash is disposed as landfill. Economics and environmental regulations will be the primary drivers in what disposal method an individual wastewater treatment plant uses.



Ahmed Ahmed Elserwy

Water & Environmental Consultant

Ain Shames University, Faculty of Science



METCALF & EDDY (1991). Wastewater engineering: treatment, disposal and reuse.



أسس التحكم فى المعالجات البيولوجية بالحمأة النشطة لمياه  الصرف



تعتمد المعالجة البيولوجية لمياه الصرف الصحي علي التحكم في العوامل المؤثرة علي تلك المعالجة وتوفير افضل الظروف للكائنات الحية الدقيقة للقيام بعملها في تكسير الملوثات العضوية القابلة للتحلل البيولوجي , فمثلا المعالجة بالحمأة المنشطة وهي احدي طرق المعالجة البيولوجية الهوائية لمياه الصرف بالنمو العالق تعتمد علي البكتيريا الهوائية في تكسير الاكسجين الحيوي المطلوب BOD ( المواد العضوية القابلة للتحلل البيولوجي ) ومن ثم فتوفير بيئة مناسبة لهذه البكتيريا والتحكم في التقاعلات الكيماحيوية داخل حوض التهوية هي من اسس التحكم في المنظومة البيولوجية للمعالجة ككل للحصول علي افضل كفاءة لازالة الملوثات في مياه الصرف.

  1. التحكم فى كفاءة العملية البيولوجية

للتحكم فى كفاءة العملية البيولوجية نظام المعالجة بالحمأة المنشطة الراجعة ، يجب مراعاة الأسس الآتية:

– كمية الأكسجين المذاب وكفاءة عمملية تقليب السائل المخلوط.

-نسبة المواد العضوية (F) إلى كمية الكائنات الحية (M) .

-عمر الحمأة.

-معدل الترسيب ومعامل حجم الحمأة

أ-  كمية الأكسجين الذائب وكفاءة عملية تقليب السائل المخلوط

تعتمد عملية المعالجة بالحمأة المنشطة على البكتريا والكائنات الحية الهوائية  التى يلزم لحياتها توفر الأكسجين المذاب. ولكى تضمن استمرار العملية بكفاءة يجب الاحتفاظ بمقدار 2.5 مليجرام  /لتر أكسجين ذائب فى محتويات حوض التهوية وحوالى ثلث مليجرام  /لتر  فى الحمأة  المعادة من المروق الثانوى إلى أحواض التهوية. كما يجب قراءة الاكسجين الذائب في الاماكن الصحيحة المناسبة في حوض التهوية قراءة صحيحى دقيقة.

هذا بالإضافة إلى ضرورة مراعاة  أن التهوية تعمل على التقليب الكامل لمحتويات أحواض التهوية فانسداد ناشرات الهواء فى بعض الأماكن أو تعطُل المحركات الميكانيكية تقلل من عمملية الخلط وبذلك تنخفض قدرة الحمأة المنشطة على استهلاك المواد العضوية والتجمع مع بعضها.

ب- نسبة المواد العضوية (F) إلى كمية الكائنات الحية (M) .

تتغذى الكائنات الحية على المواد العضوية فإذا توفرت كمية المواد العضوية يزداد تكاثر ونمو وحركة الكائنات الحية، وبالتالى تزداد الحاجة إلى الأكسجين اللازم لحياتها، وبالعكس إذا انخفضت كمية المواد العضوية يموت كثير من الكائنات الحية.

ونظرا  لأن معدل نمو البكتريا الحية يتوقف على كمية المواد العضوية المتوفرة فلذلك يلزم الاحتفاظ بكمية من المواد العضوية متناسبة مع كمية الكائنات الحية اللازمة لاستهلاكها، ويتم حساب هذه النسبة من المعادلة التالية:

المواد العضوية (F) / الكائنات الحية (M)= وزن المواد العضوية بالكيلوجرام   / وزن الكائنات الحية بالكيلوجرام  .

ونظراً لأن هذه النسبة هى التى تتحكم فى معدل النمو المطلوب فلذلك يجب حفظها فى الحدود التى تضمن كفاءة التشغيل وهى حوالى 0.15 إلى 0.45 وذلك فى المحطات التى تعمل بطريقة الحمأة المنشطة التقليدية.


ولذلك يجب على العاملين الانتظام فى حساب هذه النسبة، واذا كانت هذه النسبة عالية فهذا يدل على أن كمية التغذية متوفرة وبالتالى يرتفع استهلاك الأكسجين المذاب، وتكون هذه النسبة فى انخفاض لأن ذلك يدل على نقص فى كمية المواد العضوية، ثم يبدأ الجوع الذى يقتل كمية من  الكائنات الحية وينخفض معدل استهلاك الأكسجين وتقل حركة ونشاط الكائنات الحية الموجودة.

ج- عمر الحمأة

عمندما تكون الكائنات الحية حديثة التكوين فهى بذلك تكون كالأطفال صغار السن تأكل جيداً وتتميز بخفة الوزن وكثرة الحركة، لذلك يصعب ترسيبها.

بعكس ما إذا كانت الكائنات الحية باقية لمدة طويلة فى أحواض التهوية وتصبح فى حالة الكهولة فيقل استهلاكها للغذاء وتقل حركتها فلا تقدر على تكوين مجمومات وترسب بسرعة زائدة عن المطلوب.

والمطلوب لاستمرار كفاءة عملية التنقية الاحتفاظ بأكبر عدد من الكائنات الحية فى طور الشباب )عمر من 3 إلى 5 أيام(  فى حالة محطات الحمأة  المنشطة بالطريقة التقليدية. أما فى حالة استخدام طريقة التهوية الممتدة فيكون عمر الكائنات الحية من 20 إلى 30 يوم، حيث تتوافر فى هذا العمر جميع الشروط من حيث الاستهلاك الجيد للمواد العضوية كتغذية وزيادة فى الوزن والحركة البسيطة عند الترسيب، والتى تسمح باستخدامها كمرشح فى المروق الثانوى، فعندما تكون درجة الرسوب إلى القاع بطيئة يمكن لهذه المجموعات أن تلتقط معها ما يتبقى من المواد العالقة وهذا ما يجعل كفاءة التنقية تصل إلى أكثر من90 %

ولحساب عممر الحمأة نستخدم المعادلة التالية:

عمر الحمأة = وزن المواد العالقة فى حوض التهوية )كجم( / وزن المواد العالقة الخارجة من المروقات الابتدائية )كجم/ يوم(.




د. معامل حجم الحمأة

إذا أخذنا عمينة من السائل المخلوط من أحواض التهوية ووضعناها فى مخبار مدرج إلى علامة اللتر، تبدأ الرواسب فى النزول إلى أسفل وبعد 30 دقيقة من الزمن نلاحظ أن حجم ما ترسب فى قاع المخبار يختلف باختلاف العينات.

فمنها مثلا ما يصل حجمه إلى 500 مللى لتر وأخرى 250 مللى لتر، وغيرها تصل إلى 100 مللى لتر مثلا، ويدل ذلك على أن معدل الترسيب فى مدة 30  دقيقة يختلف باختلاف تكوين الحمأة.

ولشرح نتائج هذه التجربة نجد أن معدل الترسيب البطىء ) 500 مللى لتر مثلا ( بعد 30 دقيقة يدل على أن الحمأة أخف وزناً مما أعطى ترسيب 350 مللى لتر، والتجربة التى أعطت 100مللى لتر بعد 30 دقيقة تعتبر أثقلهم فى الوزن. بمعنى أن الحمأة الخفيفة تدل على أنها حديثة التكوين )صغيرة السن( والثقيلة التى ترسب بسرعة تكون كبيرة السن. ولكى نتمكن من التحكم فى معدل الترسيب يجب أن نحسب معدل حجم الحمأة الذى يمكن حسابه من المعادلة التالية وتسمى (SVI) :

معدل حجم الحمأة (SVI)= حجم الحمأة المترسبة بعد 30 دقيقة بالمللى لتر / تركيز المواد العالقة فى نفس العينة )مجم/لتر(







ويبين الشكل التالي بعض العوامل المؤثرة والمتحكمة في كفاءة المعالجة البيولوجية لمياه الصرف بالحمأة المنشطة.

أحمد أحمد السروي

إستشاري معالجة المياه والدراسات البيئية

المراجع العلمية

  • احمد السروي, المراقبة والتحكم في عمليات المعالجة البيولوجية , دار الكتب العلمية 2106.
  • أسس التحكم فى المعالجات البيولوجية باستخدام نتائج التحاليل الكيميائية, الدورة التدريبية عمن: خطوات العمل القياسية للتحاليل المعملية لمياه الصرف الصحى, كوفى – كيمونكس مصر, 2008.

Operational Variables affect Sludge Thickening by Centrifugation

Thickening [1]

                 Thickening of sludge is a process to increase its solids concentration and to decrease its volume by removing some of the free water. The resulting material is still fluid. Thickening is employed prior to subsequent sludge-processing steps, such as digestion and dewatering, to reduce the volumetric loading and increase the efficiency of subsequent processes.

                 The most commonly used thickening processes are gravity thickening, dissolved air floatation thickening, gravity belt thickening, and rotary drum thickening. Table 1 presents a comparison of these thickening processes.

              Selection of a particular thickening process sometimes depends on the size of the wastewater treatment plant and the downstream train chosen. The main design variables of any thickening process are:

  • Solids concentration and flow rate of the feed stream
  • Chemical demand and cost if chemicals are used for conditioning
  • Suspended and dissolved solids concentrations and flow rate of the clarified stream
  • Solids concentration and flow rate of the thickened sludge.

Table 1 Comparison of Thickening Methods [1]

Method Advantages Disadvantages
Gravity thickening


Least operation skill required Large space required
Low operating costs Odor potential
Minimum power consumption Erratic and poor solids concentration (2 to 3%) for WAS
Ideal for small treatment plants Floating solids
Good for rapidly settling sludge such as WAS and chemical  
Conditioning chemicals typically not required
Dissolved air flotation thickening Provides better solids concentration (3.5 to 5%) for WAS than that of gravity thickening Operating costs higher than for a gravity thickener
Require less space than a gravity  thickener Relatively high power consump
Will work without chemicals or with low dosages of chemicals Moderate operator attention requir
Relatively simple equipment components Odor potential
  Larger space requirements compared to other mechanical methods
  Has very little storage capacity compared to a gravity thickener
  Not very efficient for primary sludge
  Requires polymer conditioning for higher solids capture or increased loading


Effective for thickening WAS to 4 to 6% solids concentration High capital cost
Control capability for process performance High power consumption
Least odor potential and housekeeping requirements because of contained process Requires moderate operator attention
Less space required Sophisticated maintenance requirements
  Requires polymer conditioning for higher solids capture
Gravity belt thickening Effective for thickening WAS to 0.4 to 6% solids concentration Polymer dependent
Control capability for process performance Housekeeping requirements
High solids capture efficiency Odor potential
Relatively low capital cost Moderate operator attention required
Relatively low power consumption Building commonly required
Rotary drum thickening Effective for thickening WAS to 0.4 to 6% solids concentration Polymer dependent and sensitive to polymer type
Less space required Housekeeping requirements
Low power consumption Odor potential
  Moderate operator attention required
  Building commonly required

Centrifugal Thickening

             Centrifugal thickening is the acceleration of sedimentation through the use of centrifugal force. In a gravity thickener, solids settle by gravity. In a centrifuge, force 500 to 3000 times of gravity is applied; therefore, a centrifuge acts as a highly effective gravity thickener. [2]  

              Centrifuges are commonly used for thickening WAS. Primary sludge is seldom thickened by centrifuges because it commonly contains abrasive material that is detrimental to a centrifuge. In addition to being very effective for thickening WAS, centrifuges have the additional advantages of less space requirement and the least odor potential and housekeeping requirements because of the contained process. However, capital cost and maintenance and power costs can be substantial. Therefore, the process is usually limited to

large treatment plants.

                There are basically three types of centrifuges: disk nozzle, imperforate basket, and solid bowl. Figure 1 shows schematics of all three types of centrifuges. Disk nozzle centrifuges require extensive prescreening and degritting of feed sludge. They can be used on sludges with particle sizes of 400 m or less. Imperforate basket centrifuges can be used for batch operation only and not continuous feed and discharge. They suffer from high bearing wear and require significant maintenance. For these reasons, disk nozzles and imperforate basket centrifuges are being replaced by solid bowl centrifuges.

Solid bowl centrifuges (often referred to as continuous decanter scroll or helical screw conveyor centrifuges) are made in two basic configurations:

  Countercurrent and concurrent. The primary differences between the two are the configuration of the conveyor (scroll) toward the liquid discharge end of the machine, and the location and confi guration of the solids discharge port.

Sludge feed enters the bowl through a concentric tube at one end of the centrifuge.

The liquid depth in the centrifuge is determined by the discharge weir elevation relative to the bowl wall. The weir is typically adjustable. As the sludge particles are exposed to the gravitational field, they start to settle out on the inner surface of the rotating bowl. The lighter liquid (centrate) pools above the sludge layer and flows toward the centrate outlet ports located at the larger end of the machine. The settled sludge particles on the inner surface of the bowl are transported by the rotating conveyor (scroll) toward the opposite end (conical section) of the bowl. The main difference between a thickening and a dewatering centrifuge is in the construction of the conveyor and the conical part of the bowl. The slope of the conical part is less in a thickening centrifuge.

                Performance of a centrifuge is measured by the thickened sludge concentration (sludge cake concentration for a dewatering centrifuge) and the solids recovery (often called solids capture). [3]  


Operational variables [4]  

Operational variables that affect thickening include:

  • Feed flow rate
  • Feed sludge characteristics, such as particle size and shape, particle density, temperature, and sludge volume index
  • Rotational speed of the bowl
  • Differential speed of the conveyor relative to the bowl
  • Depth of the liquid pool in the bowl
  • Polymer conditioning, needed to improve performance

One of most important operational parameters of centrifuges is the factor of separation F, which demonstrates how centrifugal forces are stronger than sedimentation forces by the following equation:

F=a/g  , a=wr   or  F=r (n/g)


F =separation factor

a  =speed of centrifugal force, m/s2

g = speed of sedimentation force, m/ s2

w = angle speed of bowl (rotor), min−1

r = inside radius of bowl, m

n = speed of bowl (rotor) rotation, min−1


         Increasing the speed of bowl (rotor) rotation allows an increase in the factor of separation. However, the high bowl rotation speed can decrease the sludge particle sizes, increase the polymer demand, and decrease the effectiveness of flocculation. Therefore, centrifuges typically operate at speeds between 1500 and 2500 rpm, with the factor of separation between 600 and 1600. At the lower values of factor of separation, thickened sludge concentration and solids recovery values are lower.


Ahmed Ahmed Elserwy

Water & Environmental Consultant

Technical Manager Louts for Water Treatment



[1]   Wastewater sludge processing / Izrail S. Turovskiy, P. K. Mathai, John Wiley & Sons, Inc., Hoboken, New Jersey,2006, p 81.

[2]     L. K. Wang, Y. T. Hung, and N. S. Shammas (eds.), Physicochemical Treatment Processes. Humana Press, Totowa, NJ (2005) ,p 684.

[3]   Metcalf & Eddy, Tchobanoglous, G., Burton, F. L., Stensel, H. D. “Wastewater engineering: treatment and reuse/Metcalf & Eddy, Inc.”, Tata McGraw-Hill, 2003.

[4]   Wastewater sludge processing / Izrail S. Turovskiy, P. K. Mathai, John Wiley & Sons, Inc., Hoboken, New Jersey,2006, p 96.