Sludge Separation & Drying | Environmental Engineering - Civil Engineering (CE) PDF Download

SLUDGE 

• The polluted solid-liquid matter that is skimmed off or removed from wastewater during primary, secondary and tertiary treatment. 
• It contains 0.25 to 12% organic to inorganic solid content 
• Constituents – Organic material, nutrients, pathogens, metals, toxic substances 

Goals of Sludge Management 

•  Stabilize sludge  
•  Kill pathogens  
•  Decrease water content from 0.5-2% solids to 6 to 12% solids 

SLUDGE PROCESSING  

(a) Thickening (b) Conditioning, Stabilization, Disinfection (c) Dewatering (d) Drying (e) Composting (f) Incineration (g) Final Disposal 

[A] SLUDGE THICKENING 

• Thickening: Capacity of sludge to increase concentration of solid in sludge 
• Purpose:  To decrease volume 
• Benefits: – Reduces required capacity of downstream equipment
   – Reduce chemicals for conditioning
   – Reduce heat required by digesters
   – Reduce volume for transportation 
• Equipment types
   – Gravity  
   – Gravity Belt Thickener (GBT) 
   – Flotation
    – Rotary drum
   – Centrifuge 

[B] SLUDGE CONDITIONING 

• Sludge particles are negative (anionic) in surface charge
• The negative surface charge leads to electrostatic repulsive forces which hamper the settling process of the sludge particles.   
• Cationic conditioning agents minimizes the electrostatic repulsive force and starts floc formation 
• Chemical conditioning is similar to flocculation/coagulation process 

[C] SLUDGE DEWATERING 

• Mostly done in filtration type of units where solid particles from a fluid are retained on a filtering medium which allows the water to pass through it. 
• Five types of equipment
   – Belt Filter Press (18-25%)
   – Centrifuge (30-35%)
   – Recessed Chamber Press
   – Vacuum Filtration
   – Drying Beds 

[D] SLUDGE DRYING 

• Direct: Sludge in contact with heat surface, e.g. fluidized bed dryer, revolving drum dryers 
• Indirect:  There is no direct contact between heat source and sludge, e.g. Disc dryer 
• More expensive than mechanical methods such as pressing or centrifugation 
• Yields greater volume reduction and a storable free flowing and hygienic product. 
• End product can be used as  
   – fertilizer/soil conditioner in agriculture and forestry
   – fuel in cement kilns, power plants and incinerators
   – top soil, landscaping, and landfilling use.  

[E] SLUDGE COMPOSTING 

• Can be applied to either digested or non-digested sludge  
• Need to have sufficient mixture of organic matter content and water
• Carbon to nitrogen ratio: 25-30 • May be used as pretreatment to incineration
• Advantages – reduction in volume of materials to be transported for distribution in agricultural fields
   – allows the facilitation of storage  
   – easier to spread  
   – control in the nutrients in the compost  
   – compost is more hygienic than raw sludge application 
• Disadvantage
   – costly
   – requires aeration
   – requires a market 

[F] SLUDGE INCINERATION 

• A method used for drying and reducing sludge volume and weight. Since incineration requires auxiliary fuel to obtain and maintain high temperature and to evaporate the water contained in the incoming sludge, concentration techniques should be applied before incineration.  
• Sludge incineration is a two-step process involving drying and combustion after a preceding dewatering process, such as filters, drying beds, or centrifuges.  
• Multiple Hearths
   – Top
   – Drying
   – Middle
   – Incineration
   – Lower
   – Cooling 
• Flue gas – need to be treated 

[G] SLUDGE DISPOSAL 

• Agriculture: For raw and treated sludge
   – Things to consider: 
• Heavy Metal content 
• Dry solid content 

– Advantage: 

• Utilization of nutrients in soil (organics, nitrogen, phosphorus) 
• Cheaper (raw sludge)

– Disadvantage: need for storage facility (investment) 

• Landfilling  

DEWATERING FILTERS

• Filtration is the removal of solid particles from a fluid by passing the fluid through a filtering medium, or septum, on which the solids are deposited. However, the mechanical separation (filtration or clarification) of primary sludge is only partially effective as a treatment because 30 to 40 % of BOD and COD are water soluble and cannot be so removed. .
• Filtration is generally complete in 1 to 2 days and results in solids concentration as high as 15 to 20%. The rate of filtration depends drainability of the sludge, which in turn is related to the specific resistance

TYPES OF DEWATERING FILTERS [2] 

[A] Rotary drum vacuum filters (RDVF)

• The filtration, washing, partial drying and discharge of the sludge all take place simultaneously
• Process involves sucking of liquid through a moving septum to deposit a cake of solids.
• The cake is moved out of the filtering zone, washed, sucked dry, and dislodged from the septum, which then reenters the slurry to pick up another load of solids. 

Table 4.7.1. Advantages and disadvantages of rotary drum filters 

 [B] Filter press 

• It contains a set of plates designed to provide a series of chambers or compartments in which solids may collect.
• The plates are covered with a filter medium such as canvas. 
• Slurry is admitted to each compartment under pressure; liquor passes through the canvas and out a discharge pipe, leaving a wet cake of solids behind.
• During operation, when the frames are full of solids and no more slurry can enter. The press is then said to be jammed.
• Wash liquid may then admitted to remove soluble impurities from the solids. 

[C] Horizontal belt filter 

• It is suitable for coarser particles as compared to rotary-drum filters.
• Feed slurry flows onto the belt from a distributor at one end of the unit; filtered and washed cake is discharged from the other. 
• It is suitable for waste treatment as it is available in various sizes. They are available in sizes ranging from 0.6 to 5.5 m wide and 4.9 to 33.5 m long, with filtration areas up to 110 m². 

[D] Rotating-leaf filter 

• During filtration, the slurry enters, the filtrate exits, and solids are retained on leaves and covered with a filter cloth. 
• Upon completion of filtration, the washing and drying bottom closure opens. 
• The drive motor starts and rotates the stack of filter leaves.
• Centrifugal force causes the solids to move off the filter leaves, strike the inside wall of the tank and flow down to solid exit.
• Sizes are available up to 540 ft² per unit. 

[E] Deep bed filter  

• Filters with deep beds of sand, diatomaceous earth, coke, charcoal, and other inexpensive packing materials are normally used. · Without preseparation the bed becomes loaded quickly.  
• When the particle and bacteria in sizes smaller than the interstices of the bed, plus suspended BOD, are remove from the liquid, exceptional clarity is obtained.
• The dissolved substances, including dissolved BOD are not removed. 

THERMAL DRYERS 

Heat treatment followed by filtration is economical for dewatering sludge without using chemicals. Thermal drying of the sludge is economical only if a market for the product is available. Several types of thermal dryers used by the chemical process industry can be applied to sludge drying. The sludge is always dewatered prior to drying, regardless of the type of dryer selected. 

TYPES OF DRYERS 

[A] Flash dryer 

• It operates by promoting contact between the wet sludge and a hot gas stream. 
• Drying takes place in less than 10 sec of violent action, either in a vertical tube or in a cage mill. 
• A cyclone, with a bag filter or wet scrubber, if necessary, separates the solid from the gas phase
• The vapors are returned through preheaters to the furnace, minimizing odor problems. 
• A portion of the solid product is often returned to precondition the wet sludge. 
• Being of only moderate thermal efficiency, this type of furnace is appropriate only for low sludge flows and where heat is available cheaply [3] 

[B] Screw conveyor dryers 

• It uses a hollow shaft and blades through which hot gas or water is pumped.
• The heat is transferred to the sludge as it is conveyed through the dryer. 

[C] Multiple-hearth dryer 

• These are converted multiple-hearth furnaces. 
• The wet sludge can be mixed with dry product as it descends through the furnace. 
• Fuel burners are located both on top and bottom. 
• The outlet temperature of the gases is approximately 400 ⁰C, while that of the wet sludge at the upper drying levels barely exceeds 70 ⁰C

[E] Rotary dryers 

It consists of a rotating cylinder through which the sludge moves. 

• Various types of blades or flights are installed in the dryers depending on the type of material being dried. 
• Drying takes place by direct contact with heated air. 
• With a combustion temperature of 900 to 1000 ⁰C and 50 % excess air, the outlet temperature of gases from sewage sludge is around 300 ⁰C.

[F] Atomized spray dryers

• It has been used for many years in the chemical process industry. 
• Spraying solids counter-currently into a downward draft of hot gas dries although concurrent spray dryers are also used in the chemical industry.