Solid Waste Civil Engineering (CE) Notes | EduRev

Chapter 7

 SOLID WASTE

 DOMESTIC WASTEWATER:
It is wastewater that comes primarily from individuals, and does not generally include industrial or agricultural wastewater.

Primary treatment involves:
1. Screening: To remove large objects, such as stones or sticks, that could plug lines or block tank inlets.

2. Grit chamber: Slows down the flow to allow grit to fall out

3. Sedimentation tank (settling tank or clarifier): Settleable solids settle out and are pumped away, while oils float to the top and are skimmed off.
Secondary  treatment  typically  utilizes  biological  treatment  processes,  in  which microorganisms convert nonsettleable  solids  to  settleable  solids.  Sedimentation  typically follows, allowing the settleable solids to settle out.

Three options include:

1. Activated Sludge:The most common option uses microorganisms in the treatment process to break down organic material with aeration and agitation, then allows solids to settle out. Bacteria-containing “activated sludge” is continually re-circulated back to the aeration basin to increase the rate of organic decomposition.

2. Trickling Filters: These are beds of coarse media (often stones or plastic) 3-10 ft. deep. Wastewater is sprayed into the air (aeration), then allowed to trickle through the media. Microorganisms, attached to and growing on the media, break down organic material in the wastewater.  Trickling  filters  drain  at  the  bottom;  the wastewater  is  collected  and  then undergoes sedimentation.

3. Lagoons: These are slow, cheap, and relatively inefficient, but can be used for various types of wastewater. They rely on the interaction of sunlight, algae, microorganisms, and oxygen (sometimes aerated).
After primary and secondary treatment, municipal wastewater is usually disinfected using chlorine (or other disinfecting compounds, or occasionally ozone or ultraviolet light). An increasing  number of  wastewater facilities  also  employ  tertiary  treatment,  often  using advanced treatment methods.
Tertiary  treatment  may  include  processes  to  remove  nutrients  such  as  nitrogen  and phosphorus, and carbon adsorption to remove chemicals. These processes can be physical, biological,  or  chemical. Settled solids  (sludge)  from  primary  treatment  and  secondary treatment settling tanks are given further treatment and undergo several options for disposal.
Solid wastes are the organic and inorganic waste materials such as product packing, grass clippings, furniture, clothing, bottles, kitchen refuse, paper, appliances, paint cans, batteries etc., produced in society.
Which do not generally carry any value to the first users.

Solid waste management: It is associated with the control of waste generation, its storage, collection, transfer and transport, processing and disposal in a manner that is in accordance with the best principles of public health, economics, engineering, conservation, aesthetics, public attitude and other environmental considerations.

WASTE CHARACTERISTICS:
1. Physical characteristics:

a) Density: It is useful in design of sanitary landfills, storage, types of collection and transport vehicles etc. By compacting the waste, density can be increased.

b) Moisture content: It increases the weight of solid wastes and treatment cost will go up in incineration.

c) Size: Measurement of size distribution of particles in waste stream is important because of its significance in the design of mechanical separators and shredders. The physical properties that are essential to analyses wastes disposed at landfill are

i) Field capacity:  It  is the moisture content which can be retained in  waste sample subject to gravitational pull. It is a critical measure because water in excess  of  field  capacity  will  form leachate  and  leachate  can  be  a  major problem in landfills.

ii) Permeability of compacted waste: It governs the movement of liquids and gases in a landfill.

iii) Compressibility: Degree of physical changes of the suspended solids or filter cake when subjected to pressure.

2. Chemical characteristics: Knowledge of the classification of chemical compounds and their characteristics is essential for the proper understanding of the behavior of waste,  as  it  moves  through  the  waste  management  system.  The products  of  decomposition and heating values are to be used as fuel or are used for any other purpose, we must know their chemical characteristics.

(i) Lipids: Fats, oils and greases are lipids. They have high heating values about 38,000 KJ/kg which makes waste with high lipid content suitable for energy recovery.

(ii) Carbohydrates: Carbohydrates are readily biodegraded to products such as water, methane.
Decomposing carbohydrates attracts flies and rats and CO₂ therefore should not be left exposed for long duration.

(iii) Proteins: Compounds containing carbon, hydrogen, oxygen and nitrogen and consists of an organic acid with a substituted amino group (NH₂ ). The partial decomposition of these compounds can result in production of amines that have unpleasant odours.

(iv) Natural fibers: These are found in paper products, food and yard wastes and include  the  natural compounds,  cellulose  and  lignin,  that  are  resistant  to biodegradation. They are suitable for incineration. Calorific values of over- dried products are in the range of 12000-18000 KJ/kg.

(v) Plastic: They are suitable for recycling. Plastics have high heating value about 32000 KJ/kg which makes them suitable for incineration. But PVC when burnt produces dioxin and acid gas

(vi) Non- combustibles: This class includes glass, ceramics, metals, dust and ashes.

Waste generation: The processing of raw materials is the first stage when wastes are generated, and waste generation continues thereafter at every step in the process as raw materials are converted into final products for consumption.
We can reduce the amount of solid waste by limiting the consumption of raw materials and increasing the rate of recovery and reuse. There needs to be, therefore a societal change in the perception of wastes. This sounds simple. But, implementing changes in the society is difficult, unless appropriate management solutions are provided.
Waste collection: waste collection does not mean merely the gathering of wastes and the process includes, as well as, the transporting of wastes to transfer stations or disposal sites.

 Collection components: 

1. Collection points: These affect crew size and storage which ultimately control the cost of collection.

2. Collection frequency: Depends climatic conditions and locality.

3. Storage containers: Proper container selection can save collection energy, increase the speed of collection and reduce crew size.

4. Collection crew: The optimum crew size for a community depends on labor and equipment cost, collection methods, and route characteristics.

5. Collection route: The collection program must consider the route that is efficient for collection.

6. Transfer station: It is an intermediate station between final disposal option and collection point in order to increase the efficiency of the system, as collection vehicles and crew remains closer for routes.

7. Collection vehicles: The collection vehicle selected must be appropriate to the terrain, type and density of waste generation points.
Number of services/vehicle load N=

C= vehicle capacity m³
D= waste capacity
W= waste generation/ resistance/ kg/ service Time required collecting one load E: E=N  L      
L= loading time/ residence including on-route travel Number of loads /crew/day
n: The number of loads ‘n’ that each crew can collect in a day can be estimated based on the workday  length t and the time spent on administration and breaks (t₁ ), time for hauling and other (t₂  ) and collection route time (t₃ )

Administrative and break line time    
t₁ =A+B
A= admin B= break
Hauling and other travel time = n × H – f + G + J H= time to travel to disposal site empty truck & return to route f = time to return from site to route G = time to travel from staging  garage to route J = time to return from disposal site to garage.
Time spent on collection route t₃ = n ×E Length of work day = t = t₁ + t₂ + t₃

Where t is defined by work rules and equation A through D are solved for n Calculation of number vehicles and crews (K)

S= total services in the collected area F= frequency of collection number/week M= number of working days per week
Number of vehicle N=

x = number of households a single truck can Service per day W = number of workers per week.

Disposal options:
1. Uncontrolled dumping or non-engineered disposal
2. Sanitary landfill
3. Composting
4. Incineration
5. Gasification

Recycling: Recycling is perhaps the most widely recognized from of source reduction involving the process of separating, collecting, processing, marketing and ultimately using a material that would have otherwise been discarded.

Commonly recycled materials: Paper and cardboard, glass, metals, plastic, bacteria and tyres.
Recovery  of  biological  conversion  products: Composting  is  the  bio-chemical degradation of the organic fraction of solid waste material having a humus-like final product that could be used primarily for soil conditioning.

Bio gasification: Biogas is a mixer of gases composed of methane ( CH₄) 40-70%,  CO₂ 30-60%, other gases 1-5% including H₂, O, H₂S. It originates from bacteria in the process of bio-degradation of organic material under anaerobic conditions. Incineration and energy

Recovery: Incineration is a chemical reaction in which carbon, hydrogen and other elements in the waste mix oxygen in the combustion zone and generates heat. Energy recovery in the form of steam, which is used either to drive a turbine to generate electricity or directly for heating or coaling.