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Biowaste Treatment And Sanitation Systems

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How Does Anaerobic Digestion Work?

Anaerobic Digestion of Biowaste

These reactors contain complex microbial communities that break down (or digest) the waste and produce resultant biogas and digestate (the solid and liquid material end-products of the AD process) which is discharged from the digester.

Anaerobic digestion is a process through which bacteria break down organic matter—such as animal manure, wastewater biosolids, and food wastes—in the absence of oxygen

Anaerobic Digestion

It takes place in a sealed vessel called a reactor, which is designed and constructed in various shapes and sizes specific to the site and feedstock conditions

The following figure illustrates the flow of feedstocks through the AD system to produce biogas and digestate

Biogas

Biogas

Composed of methane (CH4), which is the primary component of natural gas, at a relatively high percentage, carbon dioxide (CO2), hydrogen sulfide (H2S), water vapor, and trace amounts of other gases

The energy in biogas can be used like natural gas to provide heat, generate electricity, and power cooling systems, among other uses

Biogas can also be purified by removing the inert or low-value constituents (CO2, water, H2S, etc.) to generate renewable natural gas (RNG)

Can be processed further to generate alternative transportation fuel, energy products, or other advanced biochemicals and bioproducts.

Digestate

The residual material left after the digestion process. It is composed of liquid and solid portions. These are often separated and handled independently, as each have value that can be realized with varying degrees of post processing.

Can be used in many beneficial applications, such as animal bedding (solids), nutrient-rich fertilizer (liquids and solids), a foundation material for bio-based products (e.g., bioplastics)

Digestate

Anaerobic System Design and Technology

Design and Technology

Anaerobic System Design and Technology

Organic Waste Collection System

1

Source segregation

Actions taken at the point of waste generation to keep and store certain materials (in this case organics) separately from other waste

Sorting at the AD plant

The organic fraction is either manually or mechanically separated from the other waste streams as part of the pre-treatment process before being fed to the digester

Sorting

Most feedstocks require pre - treatment prior to digestion

Feedstock pre - treatment

includes sorting (if not already done at source), reduction in particle size and addition

of water before the mixture is fed into the AD system

Pre-treatment and feeding procedure

Enhance

degradation of volatile solids

increase biogas yield

Avoid problems with digestion

water helps control the total TS fed to the

digester

Anaerobic Digester Designs

The digester is designed to provide the optimal conditions for converting the organic waste into biogas.

A batch digester is the simplest form of digestion, where manure is added to the reactor at the beginning of the process in a batch and the reactor remains closed for the duration of the process.

induced Blanket Reactors are digesters in which a blanket of sludge develops and retains anaerobic bacteria, providing a bacteria-rich environment through which the feedstock must pass.

Fixed film digesters contain plastic media (e.g., pellets) on which bacteria attach and grow, instead of relying solely on suspended bacteria to break down the digester feedstock.

2

The operating parameters of the digester enhance the microbial activity and thus increase the AD efficiency

Temperature - Mixing - Inhibition - pH

Carbon to nitrogen ratio

Inoculation and start-up

Organic Loading Rate

Hydraulic Retention Time

Operational parameters

Anaerobic digesters can be fed continuously or batch-wise.

In a continuous feeding

mode, new feedstock is added at regular intervals while an equivalent volume of slurry

leaves the digester, thereby providing a continuous process of digestion

In batch-fed digesters, the reactors are filled with a feedstock, closed and left for a period of time (i.e. the retention time), then opened again and emptied - cheaper

Feeding Mode

(wet / dry systems)

Depending on the TS content of the substrate fed into an AD system, digester designs are defined as either wet or dry systems

Total solids content

Operating temperature

(mesophilic / thermophilic)

Mesophilic systems are considered more stable and require less energy

HOWEVER

Higher temperature of the thermophilic digestion systems facilitates faster reaction rates and faster gas production

Biogas: Processing, Transport, And Use

4

Electricity and Heat

Biogas is most often used to generate electricity.

Thermal energy in the form of waste heat, produced during electricity generation, can be recovered to heat digesters or adjacent buildings.

Biogas can be fired directly in boilers or heaters as a replacement for propane

Electricity and Heat

Transportation Fuel

Transportation Fuel

Biogas can be processed to pipeline quality and sold to the local gas utility.

Biogas can be converted to compressed natural gas (CNG) which can be used in applications such as vehicle fuel.

Biogas storage

Must be UV-, temperature- and weather-proof

Must be gas

tight and pressure-resistant

Biogas generation varies during the day according to feeding patterns and ambient temperature changes

In addition, gas production continues during the night

Biogas generation and consumption often do not happen at the same

time. It is thus necessary to collect the produced biogas temporarily in appropriate storage facilities.

Biogas can be stored in a gastight container for long periods of time

without losing its energy content

3

TYPES

The size of the gas storage container is determined by the rate of gas production

and rate of biogas usage. The easiest way to store biogas is in low-pressure systems

such as a floating-drum, a fixed-dome or in a gas storage bag (balloon), all of which

are used in developing countries.

Floating-drum

The drum serves as the gas storage

facility. The produced gas fills the drum and pushes it upwards. The more gas that is produced, the higher the drum rises.

Low-pressure storage systems

Fixed-dome

The upper part of a fixed-dome digester serves as a gas storage facility and it is therefore essential that it is gas-tight to prevent gas leaks. When the exit

valve is closed and gas accumulates in the dome (e.g. during night), the gas pressure will increase and push the slurry in the digester downwards and out into the compensation tank

Figure 13: Fixed-dome digester without (left) and with (right) gas pressure [2].

Gas storage tank

Due to the resulting higher energy density, it needs

less space than the low -pressure options

With a compression to 10 bar, 10 times more biogas can be

stored than at normal pressure. The compression

in this pressure range can be achieved with a single stage compressor.

A pressure regulator is then however

needed for withdrawal of gas.

Medium-pressure storage

Gas tank in India with 10 bar

pressure (photo: Sandec).

Gas bottle /cylinder

Storage of biogas under high-pressure (i.e. compression to more than 200 bar) is technically

feasible in special gas bottles.

It is essential to purify the gas and remove hydrogen and H2S which lead to corrosion of the gas bottle

Car engines can be operated with

high-pressurized biogas

This option is only feasible for large-scale biogas plants due to the high costs

High-pressure storage

Conditioning of biogas

When biogas leaves the digester, it is saturated with water vapor and contains high

amounts of energy-deficient CO2 and varying quantities of corrosive and toxic hydrogen

sulphide (H2S).

Depending on the use of the biogas, it may have to be cleaned to remove the H2S or water vapour.

This is particularly important when using a gas-driven engine to produce electricity.

Dewatering

Biogas that leaves the fermenter is nearly 100 % saturated with water vapour.

Water vapour can lead to corrosion of the energy conversion equipment and therefore has to be removed from the biogas

When biogas moves through the gas pipelines from the digester to the conversion equipment, the vapour cools down on the walls of the sloping pipes and condenses.

Using the System Templates

Using the System Templates

A System Template defines a suite of compatible

technology combinations from which a system can be designed.

Selecting sanitation options using the system templates

STEPS:

Identify the products that are locally generated and/or available THEN the system templates that process the defined products

For each template, select a technology from each

functional group where there is a technology

choice presented

SELECTING SANITATION OPTIONS

Compare the systems and iteratively change individual technologies or use a different system template based on user priorities, the demand

for specific end-products

EXAMPLE

Products are materials that are also called ‘wastes’ or ‘resources’.

Some products are generated directly by humans (e.g., Urine and Faeces), others are required in the functioning of technologies (e.g., Flushwater to

move Excreta through sewers) and some are generated as a function of storage or treatment

Products

FIGURE

TYPES

A functional group is a grouping of technologies that have similar functions

Functional Groups

Table

User interface

U

Different designs for squatters, sitter, washers,wipers

Water based and waterless models

The choice influence the subsequent technology

An inclusive design of user interface allows barrier-free access for all users

Availability of water for flushing

Habits and preferences of the user

Local availability of the material

Compatibility with subsequent collection and storage/treatment or conveyance technology

How to choose the right one

Collection and storage/ Treatment

s

Function in collecting, storing, and sometimes treating the products generated at the user interface

The treatment provided by these technologies

is often the function of storage, and is usually passive, without requiring energy input.

Products that emanate from these technologies

often require subsequent treatment before use or disposal.

1-A single pit technologies

2-Double pit technologies

3-Anaerobic technologies

Technologies

1

Hole in the ground

Three or more meters deep.

It normally has a lining to stabilize

the walls of the pit.

The user interface can then be

placed on top of it

PROS

Can be at very low cost

Most suited to rural area

PROS & CONS

CONS

Emptying costs may be quite high

The sludge needs to be further treated.

Emptied sludge is still very pathogenic

leakage can contaminate groundwater resources.

Odors are normally noticeable

FIGURE

Ventilated Improved Pits VIP

Ventilated

Pit

DOUBLE VENTILATED IMPROVED PIT

2

Longer life span

easier emptying

significant reduction in pathogens

product can be used as soil conditioner

However

Manuel removal of humus is required

possible contamination of groundwater

higher costs

FIGURE

The pits are dug to a maximum depth of 1.5 meters,

and therefore, often do not require a lining on the entire side walls allowing for soil organisms to enter and do their job.

Fossa

Alterna

Designed to make higher quality earth-like product

that can be used as a valuable soil conditioner.

This is achieved by adding soil, ash, or leaves

as cover material after each defecation which absorbs the liquids inside the pit and increases the pore space allowing for aerobic conditions.

Greywater should not be added because it fills the pore spaces

and deprives the aerobic bacteria of the oxygen that is required for degradation.

PONS & CONS

Are designed to receive blackwater as input and possibly also greywater.

The pour flush toilet is used as user interface and connected to one of the pits by a pipe.

When the pit becomes full, it is closed,

and allowed to rest before being emptied.

As this is a water-based wet technology

the full pits require a longer retention time

of two years to degrade the material.

Over time, the content is sufficiently dewatered

and can be safely excavated in the form of pit humus

Twin pits for pour flush

FIGURE

PONS & CONS

The word 'anaerobic' describes biological processes that occur in the

absence of oxygen.

These processes naturally take place in swamps, marshes, ponds and other

standing water bodies.

3

SEPTIC TANK

Blackwater and greywater enter the tank through an inlet T.Settleable solids fall down to the bottom, where they accumulate as sludge.

With time, the sludge undergoes some stabilization by anaerobic digestion.

It has to be removed every 3-5 years.

Floatable substances move up to the surface, forming a scum layer.

The effluent from the clearwater zone, in the middle, flows out through an outlet T and is then typically are infiltrated

It is a water-tight tank, typically made of reinforced concrete,

polyethylene or fiberglass and,

It has an outlet, through which the clarified effluent leaves the tank.

septic tanks are passive and do not require

electrical energy to function Construction is relatively costly but the technology has a long service life and little place is required

PROS & CONS

Anaerobic baffled reactor

Equipped with baffles,which form several anaerobic chambers to remove and digest organics and improve the treatment performance

Anaerobic filter

The treatment can be further improved by providing 1-3 fixed-bed filter units, which helps to trap non-settleable suspended particles and provide an increased surface for microbial growth

Low operation and maintenance costs

No electrical energy is required

Resistant to organic and hydraulic shock loads.

Can be built underground

Advantages

Biogas Reactor

Anaerobic digestor is designed to generate biogas and to capture and collect it, so that it can be used as a renewable energy source for cooking or other purposes.

The blackwater from a toilet can be directly connected to the digestor

In order to produce significant amounts of biogas, additional inputs like cow dung or

organic solid waste, are normally mixed in at the inlet.

The substrate flows into the reactor, where it has a retention time of, typically, 15-to-25 days.

The biogas collects in the dome at the top of the reactor, and is transported to the point of use, through a gas pipe. When new substrate is added to the reactor

or when the gas pressure rises, the digestate flows into the expansion chamber from where it can be removed

PROS & CONS

Most efficient and compact.

They work at best in warmer climates so are particularly appropriate for tropical areas.

Require very little operation and maintenance.

Advantages

• Availability of space

• Soil and groundwater characteristics

• Type and quantity of input products

• Local availability of materials

• Desired output products

• Availability of technologies for subsequent transport

• Financial resources

• Management considerations

• User preferences

Depends on

Conveyance Technologies

C

What happens when the pit is full?

The sewered systems which rely on on pipe networks

The non-sewered systems, relying on different kinds of container-based solutions

The non-sewered systems

Overflowing

Manuel emptying

Motorized emptying

Manuel emptying

Human-powered emptying of pits, vaults, and tanks can be done in two ways: either with buckets and shovels or using a portable, manually operated pump

Motorized emptying refers to vehicles equipped with a motorized pump and a storage tank.

Motorized emptying

PROS & CONS

Three categories of sewer systems

CHOICE DEPENDS ON PRICE AND MAINTENANCE REQUIREMENTS

The sewered systems

The simplified sewer system

The solids-free sewer system

Conventional gravity sewers are large networks of underground pipes that convey black water, gray water, in many cases stormwater, from the houses to the wastewater treatment plant,

using gravity and often pumping stations.

Conventional gravity sewer system

The type and quantity of product to be transported

The distance to cover,

The accessibility,

Topography,

Soil and groundwater characteristics,

The financial resources,

The availability of service providers,

Management considerations.

DEPENDS ON

The centralized

and semi-centralized treatment technologies.

The pre-treatment

The primary treatment

The secondary treatment

The Tertiary treatment

T

TREATMENT STAGES

Primary treatment

consists of liquid/solid separation

Secondary treatment

consists of the removal

of organic matter and suspended solids.

The pre-treatment

the preliminary removal

of waste water or sludge constituents, such as oil,

grease and various solids, like sand and trash.

Tertiary treatment

consist of removing elements like remaining pathogens,

nutrients such as nitrogen and phosphorous,

TREATMENT STAGES

PRE-TREATMENT

Settler

remove suspended solids by sedimentation.

The low flow velocity in the settler

allows sediment particles

to sink to the bottom,

while the lighter constituents

will float to the surface,

forming the scum.

Settlers can achieve an initial reduction of up to 50% to 70% of suspended solids,

and up to 20% to 40% of organic material.

Settlers can be independent units,

or merged into other technologies.

the flow tank,

or settling compartment,

is designed with a "V" shape,

which prevents gas bubbles to come up

and fold them to the sides, as well as the scum.

Imhoff Tank

Combines solid liquid separation and digestion of the septic sludge.

Because there is sludge digestion,

there is production of biogas.

whose bubbles may disturb the settling process.

POST-TREATMENT

Upflow Anaerobic Sludge Blanket

USAB

Wastewater enters the reactor from the bottom,

and flows upwards.

A suspended sludge blanket

filters and treats the wastewater

as it flows through it.

The sludge blanket is comprised of

small agglomerations of microorganisms

with a diameter of about 1 to 3 mm,

which because of their weight, resist being washed out in the upflow.

Aaerobic Technologies

Aerobic systems are biological systems which rely on the action of bacteria needing oxygen to live, like us.

Aaerobic Technologies

Waste deposition ponds

WSP

WET LANDS

Aim to replicate the naturally occurring processes of a natural wetland, marsh or swamp.

WET LANDS

Be Aware !!

Risk of clogging

Management of the plants,

High ammonia levels

Free Water Surface

The water slowly flows through the wetland particles settle, and pathogens are eliminated through the combined action of sun, settling, adsorption, and predation from higher organisms.

It is only appropriate for low strength wastewater.

Horizontal Subsurface Flow

As wastewater flows horizontally through the basin,

the filter material filters out particles.

It acts both as a filter and as a fixed surface

upon which bacteria can attach, forming large biofilms

The plant roots play an important role

in maintaining permeability in the filter.

Most sophisticated and performance type of wetland

Vertical Flow

The water flows down and is collected by a drainage system.

The wastewater is applied intermittently,

four to ten times a day.

Thus, the filter goes through stages of being saturated and unsaturated,

Higher End Technologies

Performant in developed countries,

BUT imply high capital costs,

operation and maintenance by skilled personnel,

and a constant source of energy.

Higher End Technologies

Organics are degraded by the biofilm covering the filter material.

Trickling Filter

Small land requirement;

however, flies and odors are often problematic.

Operates mostly under aerobic conditions.

After primary treatment, wastewater is continuously trickled or sprayed through the filter,

Activated Sludge System

It makes use of highly concentrated micro-organisms

to degrade organics and remove nutrients,

leading to a high quality effluent.

To maintain aerobic conditions

and to keep the activated sludge suspended,

a continuous and well-timed supply of oxygen is required.

The four main functions of sludge treatment

The solid-liquid separation

Stabilisation

Dewatering or drying,

Pathogen reduction.

Sludge Treatment

Sludge is a highly concentrated

and variable material.

Before constructing a faecal sludge treatment plant, it is necessary

to make a good assessment of the initial situation in the city

and estimate the characteristics and quantities of the sludge to treat.

Potential beneficial end uses

dry side as a fertilizer

dry side as a fueL or biogas,

USES

This technology is relatively low-cost

but requires a large land area,

if possible, far from habitations,

as it generates odors and flies.

Sedimentation Thickening Ponds

solid-liquid separation

Such ponds allow the sludge to thicken.

As the sludge settles and digests,

the supernatant must be decanted and treated separately.

The thickened sludge can then be dried or further composted.

Planted drying beds

The plants and their root systems maintain the porosity

of the filter, which means that sludge can be added

layers after layers, which imply also a much longer retention time.

FIGURE

Co-composting

sludge is added to organic solid waste

In open composting, such as shown in this figure, the mix material

is piled into long heaps called "windrows"

and left to decompose.

The piles are periodically turned

to provide oxygen and ensure a homogenous treatment.

D

Use or Disposal

-Agricultural Use

(as fertilizers)

-Use and/or disposal of effluent and other products

Fill and cover / Arborloo

Technologies

for using or disposing of treated effluent

Effluent is the general term

for a liquid that leaves the technology,

typically after blackwater or sludge has undergone solid-liquid separation

or some other type of treatment.

Utilization of effluent

Drip irrigation where the water is slowly dripped on or near the root area

Surface irrigation where water is routed over land

Irrigation

Properly treated wastewater can be used for irrigation in agriculture

Aquaculture

Aquaculture refers to the controlled cultivationof aquatic plants and animals.

Biogas

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Sanitation as a multi-step

process in which human excreta and wastewater are managed from the point of generation to the point of use or ultimate disposal

A Sanitation System is a

context-specific series of technologies and services for the management of these wastes

i.e., for their collection, containment, transport, transformation, utilization or disposal

A sanitation system

is comprised of Products (wastes) that travel through Functional Groups which contain Technologies that

can be selected according to the context

Sanitation

A sanitation system also includes

the management, operation and maintenance (O&M)

  • Improve public health
  • Promote social development
  • Enhance attractiveness of the city
  • Strengthen the economy

Importance

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