19 Sludge Treatment And Disposal

Meenakshi Nandal

Objectives:

Understand the process of sludge production

Understand the process of management process Explain various types of sludge treatment stages Understand the final disposal of sludge

Introduction:

In today’s era, a strategic approach towards global impact on the environment must be developed and if this aspect is elapsed, a change of environmental loads or their effect will be caused and no reduction will be attained. For instance, a wastewater treatment plant (WWTP), which is a main concern for ecological treatment system, gives rise to an environmental impact due to its energy consumption, use of chemical compounds, emissions to the atmosphere and sludge production. The main solid by-products produced in wastewater treatment are:

• screened material
• grit
• scum
• primary sludge
• secondary sludge
• chemical sludge (if a physical–chemical stage is included)

The residue that accumulates in sewage treatment plants is called sludge (or biosolids). Sludge is known to be the largest by-product from treatment of wastewater and its disposal is one of the most challenging environmental problems. Sludge is a by-product of water and wastewater treatment operations. Sludge produced from biological treatment operations may be referred to as wastewater biosolids. Sludge is needed to be treated to a certain degree before it can be disposed off. The type of treatment needed depends on the disposal method proposed. There are principally three final disposal strategies for wastewater sludge and sludge components even though there are many “grey zones” between these are clear-cut alternatives. Sludge and sludge components may be deposited on land (in landfills or special sludge deposits), in the sea (ocean disposal) or to a certain extent in the  air (mainly as a consequence of incineration). The solids in the sludge contain nutrients which may be valuable to plants, as well as humus like material which improves the capacity of poor soils to hold water and air. Unfortunately, industrial sources, including household wastes and urban runoff, introduce quantities of toxic materials into municipal sludge. Human waste also contains harmful organisms, disease-causing bacteria, viruses and parasites.

Sewage sludge consists of the organic and inorganic solids that were present in the raw waste and were removed in the primary clarifier, in addition to organic solids generated in the secondary/biological treatment and removed in the secondary clarifier or in a separate thickening process. The generated sludge usually is in the form of a liquid or semisolid, containing 0.25 to 12 percent solids by weight, depending upon the treatment operations and processes used. Treatment and disposal of sewage sludge are major factors in the design and operation of all wastewater treatment plants. Two basic goals of treating sludge before final disposal are to reduce its volume and to stabilize the organic materials. Stabilized sludge does not have an offensive odour and can be handled without causing a nuisance or health hazard. Smaller sludge volume reduces the costs of pumping and storage.

The problems of dealing with sludge are complex because:-

1) It is composed largely of the substances responsible for the offensive character of untreated waste water.

2) The portion of sludge produced from biological treatment requiring disposal is composed of the organic matter contained in the waste water but in another form, and it too, will decompose and become offensive.

3) Only a small part of the sludge is solid matter.

20.1 Sludge Treatment Stages:

The main stages in sludge management, with their respective objectives are:

Thickening: removal of water (volume reduction)

Stabilization: removal of organic matter – volatile solids (mass reduction)

Conditioning: preparation for dewatering (principally mechanical)

Dewatering: removal of water (volume reduction)

Disinfection: removal of pathogenic organisms

Final disposal: final destination of the by-products

• 1) Thickening: A physical concentrating of the sludge for minimizing its water content

• 2) Stabilization: It is the removal of biodegradable organic matter of the sludge and hence reduction in in the solids mass in the sludge.

• 3) Conditioning: It is a process in which sludge dewatering capacity is improved for capturing of solids in the sludge dewatering systems by adding of chemical products (coagulants, polyelectrolytes

• 4) Dewatering of sludge can be done through natural or mechanical methods. In this phase further removal of water is done as it has an important impact in its transport and final disposal costs, besides influencing its subsequent handling, since the mechanical behavior varies with the water content level.

• 5) Disinfection: It is a very important part and is to be done if the sludge is used for agricultural recycling as the anaerobic or aerobic digestion processes do not decrease the pathogens contents. If the sludge is to be incinerated or disposed of in landfills, then disinfection is not necessary. Below is the table showing the sludge management stages and main process used.

Fig 20.1: Sludge management process and flow sheet

20.1.1 SLUDGE THICKENING:

The main processes used for sludge thickening are:

• gravity thickeners
• dissolved air flotation
• centrifuges
• belt presses

Gravity thickeners: They have a identical structure to settling tanks. It is usually circular having central feeding, a bottom sludge exit and a supernatant side exit. The thickened sludge is used for the next stage i.e. stabilization), while the supernatant is returned to the head of the works.

Dissolved air flotation: With high pressure the air is introduced in a solution and in such conditions air gets dissolved. The dissolved air is released in the form of small bubbles when there is depressurization. The sludge particles are carried to the surface with the upward movement of bubbles and hence removed.

Thickening by flotation: It has a good applicability for activated sludge which does not thicken well in gravity thickeners.

Dissolved air flotation: It also has good applicability in wastewater treatment plants with biological phosphorus removal, in which the sludge needs to remain in aerobic conditions in order not to release the phosphorus into the liquid mass.

20.1.2 SLUDGE STABILISATION:

Raw sewage sludge is rich in microorganisms, decomposes easily and quickly releases offensive odours. With the objective of digesting the biodegradable fraction of the organic matter present in the sludge, the stabilization processes were developed, thus decreasing the risk of putrefaction, as well as reducing the concentration of pathogens. The stabilization processes can be divided into:

• Biological stabilization: The stabilization of sludge is done by using specific bacteria to promote the stabilization of the biodegradable fraction of the organic matter
• Chemical stabilization: the stabilization of the sludge is achieved by the chemical oxidation of the organic matter.
• Thermal stabilization: obtained from the action of heat on the volatile fraction in hermetically closed recipients.

The most frequently used sludge stabilization process is anaerobic digestion. Aerobic digestion is less diffused, but has a good applicability in the stabilization of the excess activated sludge originating from WWTPs with biological nutrient removal. Composting processes are very common in urban areas, but it is limited in very less number of small-scale wastewater treatment plants. The other used processes are alkaline treatment and thermal drying.

Table 20.1 Sludge stabilization technologies and final disposal methods

The process of anaerobic digestion has been known by sanitary engineers since the end of the 19th century and is characterized by the stabilization of the organic matter in an environment free from molecular oxygen. Due to its high efficiency, anaerobic digestion is used in simple domestic septic tanks, up to completely automated plants and for serving large metropolitan regions. In a conventional activated sludge or trickling filter plant, the mixture of primary sludge and excess biological sludge is stabilized under anaerobic conditions and can be converted into methane (CH4) and carbon dioxide (CO2). Anaerobic digesters are employed for anaerobic digestion. The digester is fed in a continuous or batch form and certain detention time is maintained. The anaerobic digesters are made up of concrete or steel. The raw sludge is mixed and heated in closed chamber and the gas is produced and the gas is stored in floating gasholders for processing or burning in temperate climate countries. According to land availability, the need for maintaining a completely-mixed regime and the removal of grit and scum, the configuration of the digesters is differentiated.

20.1.3 SLUDGE DEWATERING:

Digested sludge is used for dewatering and it has an important impact on the sludge transportation and final disposal costs. The main factors for sludge dewatering are:

• the transportation cost to the final disposal site is reduced;
• improvement of the handling conditions and transportation of the sludge
• the reduction of the water increase the calorific value of the sludge aiming at preparing it for incineration;
• disposal in a landfill or for agricultural use may get a reduced volume;
• minimization in the production of leachate when the sludge is disposed of in landfills. Natural or mechanised processes are generally used for the sludge dewatering. Natural processes use evaporation and percolation for dewatering. Although they are very simple and cheap to operate but need larger areas and volumes for installation. In contrary, mechanized processes are based filtration, compaction, or centrifugation to accelerate dewatering, resulting in compact and sophisticated units, from an operational and maintenance point of view.

The main sludge dewatering processes are listed below:

Natural

• Drying beds
• Sludge lagoons

Mechanised

• Vacuum filters
• Belt presses
• Filter presses
• Centrifuges

For increase of dewatering capacity and the solids capture, the sludge is incorporated to a conditioning stage before starting of the dewatering stage itself. Chemical products or physical processes are used for conditioning and for physical process, the most common is heating of thesludge. The chemical products are applied to the sludge favors the aggregation of the solids particles and formation of flocs. The conditioning can be also employed upstream of the mechanised thickening units. The very common coagulants used are metallic salts and polyelectrolytes (polymers).

The most common metallic coagulants are:

• Aluminium sulphate
• Ferric chloride
• Ferrous sulphate
• Fferric sulphate
• Quicklime/ hydrated lime

1. a) Sludge drying beds: It is one of the earliest techniques and excessively used for solids-liquid separation in sludge. In comparison with mechanical dewatering options the construction cost is generally low, specifically for small-sized commodities. The setup generally having a rectangular tank with concrete walls and a concrete bottom. In the inside of the tank, these are the following devices to drain the water present in the sludge:

• support layer (bricks and coarse sand), on top of which the sludge is placed
• draining medium (fine to coarse sand followed by fine to coarse gravel)
• drainage system (open or perforated pipes)

Some portion of the liquid evaporates and some percolates through the sand and support layer. The dewatered sludge remains in the layer present above the sand. Drying beds are suggested for small and medium sized communities with a population of around 20,000 inhabitants.

1. b) Sludge drying lagoon: These are used for thickening, integral digestion, dewatering and even for the final disposal of sewage sludge. Generally excavated in the soil, drying lagoons are located in natural depressions in the land, or put inside banks. The discarded sludge is kept for long time periods generally from 3 to 5 years. The sludge is condensed by the action of gravity and further digested by the microorganisms present in the sludge and further it is dewatered through drainage. This process is advocated for dewatering the earlier digested sludge by aerobic or anaerobic

processes, but not for the dewatering of primary or mixed sludge. Sludge lagoons are much less  used than drying beds among the natural dewatering processes. The main difference between these two process is in the fact that evaporation is the principal mechanism of influence in the dewatering process.

Fig 20.4. Sludge drying bed

Source: image.slidesharecdn.com/environmentalengineering project

Percolation has a lesser effect than in the drying beds. The dewatering of the sludge in the lagoon can be done with the use of devices for the removal of the supernatant water at various levels after the loading of the sludge. But use of drains at the bottom is not a very common practice in drying lagoons, as the sewage sludge has lessened the ability of draining and there may be the risk of pipe clogging.

Table 20.2 Advantages and disadvantages of sludge drying beds

As soon as the lagoon gets filled to the top, it can be put out of operation without the removal of the sludge, thus serving as a solution for final disposal. Another feasible solution is the removal of the sludge from the fully- filled lagoon and which is then allowing to reuse and utilization as a continuous dewatering unit.

Centrifuge: It is the solid or liquid forced by the action of a centrifugal force. In a first stage, the sludge particles settle down at a velocity which is much higher than the action of gravity. Sludge loses its part of the capillary water under the continuous action of centrifugation and compaction takes place. After this last stage of dewatering, cake is removed

Fig 20.6. A Centrifuge Source: upload.wikimedia.org/wikipedia/commons/c/cb/Solid_Bowl_Centrifuge_3.png

One can say that, centrifuges are equipment that are be used for sludge thickening and dewatering and it is possible to place the centrifuges in series, the first for the thickening of the sludge and the second for the dewatering. Vertical and horizontal-shaft centrifuges are the main types of centrifuges used for sludge dewatering. The type of feeding of the sludge, the intensity of the centrifugal force and the manner in which the cake and the liquid are unloaded from the equipment are the main factors considered for differences in the process.

1. d) Vacuum filter: Vacuum filters were mainly used in industrialized countries for sludge dewatering until the 1970s. Their use entered into decline due to the high-energy consumption and lower efficiency when compared with modern sludge dewatering processes. A vacuum filter is made up of a rotating cylindrical drum which is installed with partial submergence in a tank with conditioned sludge.

Estimately 10- 40% of the drum surface is kept submerged in the tank and this part performs the filtration or you can say cake formation zone. The cake formation takes place in the outer part of the cylinder, while the filtered liquid drained to the interior, where there is a vacuum. In next stage, there is a dewatering region that occupies between 40-60% of the cylinder surface in the direction of the rotation. In the final region of the cylinder, there is the unloading zone. A valve takes the surface of the cylinder to the atmospheric pressure in this region and the sludge cake is then separated from the filtering medium.

1. e) Filter press: A filter press operates in a fitful mode having cycles consisting of sludge loading, filtration and cake unloading stages. The liquid sludge is pumped into plates which are surrounded by filter cloths. The pumping of the sludge in the space between the plates increases the pressure and encourages the sludge to pass through the filter cloth.

The cake is formed as the solids are retained on the filtering medium. Next, a hydraulic piston pushes a steel plate against the other polyethylene plates, making up the pressing.

The filtrate (liquid) filters goes through the filter cloths and is being possessed by the plate outlet ports. When the pneumatic piston is retreated, cake is easily removed from the filter and the plates are separated. Here, the dry cake falls from the plate and can be used for storage or final destination.

                                                                                      Fig 20.9: Process of Belt press

1. f) Belt press: Belt presses work on a continuous mode system. The process can be categorized into three different stages or zones: (a) zone of gravity drainage, (b) low-pressure zone and (c) high-pressure zone.

In zone of gravity, drainage is constructed at the entrance of the press where the sludge is applied onto an upper screen and under the action of gravity the free water percolates through the opening pores present in the screen.

After this step, the rest of the free water is removed as the sludge is directed to the low-pressure zone and the sludge is gently compressed between the upper and lower screens. Finally, scrapers remove the sludge and high-pressure water jets wash the screens. Low acquisition costs and reduced energy consumption are the main advantages.

20.1.4 SLUDGE DISINFECTION: The main objective of introduction of sludge disinfection stage is lowering of pathogens in the sludge and when it is disposed off, it may not cause health risks to the population and to the workers who will handle it and also negative effects to the environment. However, the need to adopt a disinfection system will totally depend on the final disposal alternative to be adopted. Higher sanitary level is required if the sludge is applied in public parks and gardens or its recycling in agriculture. These requirements can be met by a sludge disinfection process or by temporary restrictions to public use and access. The most important processes are described below.

1. a) Composting: Composting is a decomposition of organic matter aerobically which can be achieved through controlled conditions of temperature, water content, oxygen and nutrients. The resultant product i.e. compost can be used as a soil conditioner and has high agricultural value. The inactivation of the pathogens is done mainly by the increase of temperature during the highest activity phase of the process. The temperature can reach between 55-65 ◦C due to of biochemical reactions.

Both the raw sludge and digested sludge can be composted. Materials like wood chips, leaves, green waste, rice straw, sawdust need to be added to the sludge which helps in improving the water retention capacity, increase the porosity and balance the ratio between carbon and nitrogen. The three methods are employed for this are:

• Windrows. Periodical turning, in order to allow aeration and mixture. Detention time must be between 50 and 90 days.
• Aerated static pile. Aeration done by perforated pipes from air blowers or exhausting systems. Detention time must be between 30 and 60 days.
• In-vessel biological reactors. Closed systems, with a greater control and lower detention time. Detention time must be at least 14 days in the reactor and 14 to 21 days for cooling.

1. b) Thermophilic aerobic digestion: The process of thermophilic aerobic digestion also termed as autothermal  digestion  works on the  same principles as  of conventional aerobic  digestion

system. The difference is that it operates in a thermophilic phase due to some alterations in the concept and operation of the system.

The sludge is generally pre-thickened in this stage and is operated with two aerobic stages, and there is no need of outsourced energy to raise the temperature. In the small and closed chamber, the concentration of solids in the sludge is higher, the heat released from the aerobic reactions heats the sludge to temperatures higher than 50◦C in the first stage and 60◦C in the second. Pathogenic microorganisms are reduced to values lower than the detection limits if the sludge is maintained at a temperature between 55–60 ◦C for 10 days. The two most important factors for the operational success of the system are mixing and aeration efficiency.

12. c) Lime stabilization: A reduction of the population of microorganisms, pathogens and the potential occurrence of odours takes place when a sufficient quantity of lime is added to the sludge to increase the pH to 12. Lime can be added to liquid or dewatered sludges. Lime stabilization is used to treat primary, secondary, or digested sludge.

1. d) Pasteurisation: It involves the heating of the sludge to 70◦C for 30 minutes, followed by a rapid cooling to 4◦C. The sludge can be heated by heat exchangers or by hot steam injection. The steam injection process is more commonly used and the sludge is pasteurised in batch to decrease the recontamination risks.

1. e) Thermal treatment: Thermal treatment consists of passing the sludge through a heat source that causes the evaporation of the existing moisture in the sludge and consequently the thermal inactivation of the microorganisms. To be economically feasible, the sludge needs to be previously digested and dewatered to a solids concentration in the order of 20–35%, before being thermally treated. The dried sludge has a granular aspect and presents a very high level of solids, in the region of 90–95%.

20.1.5 FINAL DISPOSAL OF THE SLUDGE

Main final disposal activities of the sludge:

Ocean disposal: After pre-conditioning, the sewage is disposed in the sea. This type of disposal is without any beneficial use.

Incineration: Thermal decomposition process by oxidation, in which the volatile solids of the sludge are burnt in the presence of oxygen and are converted into carbon dioxide and water. The fixed solids are transformed into ashes. Disposal without beneficial uses.

Sanitary landfill: Disposal of the sludges in ditches or trenches, with compaction and covering with soil, until they are totally filled, after which they are sealed. The sewage sludge can be disposed of in dedicated landfills or co-disposed with urban solid wastes. Disposal without beneficial uses.

Land farming: Land disposal process, in which the organic substrate is biologically degraded in the upper layer of the soil and the inorganic fraction is transformed or fixed into this layer. Disposal without beneficial uses.

Land reclamation: Disposal of sludge in areas that have been drastically altered, such as mining areas, where the soil does not offer conditions for development and fixation of vegetation, as a result of the lack of organic matter and nutrients.

Agricultural reuse: Disposal of the sludge in agricultural soils, in association with the development of crops.

Summary:

In this lecture we learnt about:

• Process of sludge production
• Process of management process
• Types of sludge treatment stages
• Process of final disposal of sludge

References:

• Mahvi AH, Mesdaghinia A, Karakani F (2004). Feasibility of continuous flow sequencing batch reactor in domestic wastewater treatment. American J App Sci, 1(4): 348-53.
• Metcalf & Eddy Inc (2003). Wastewater Engineering: Treatment and Reuse. 4th ed. McGraw-Hill Inc, New York, pp.: 6-17.
• Unit operations and processes in Environmental Engineering, Reynolds and Richards, PWS Publishing company, Boston, M.A,1996.