Waste Water Or Sewage Treatment

Waste Water or Sewage treatment

Booklet No. 645

Ecology and Environment: EES-36

Preface

The first part of the booklet is a down load from internet and html format is maintained precisely because to show that it is so and also to help those would like to go back to the original internet articles. Today we can know anything from anywhere in the world in the internet and the best, the latest and the most up-to-date knowledge on waste water or sewage treatment is also available in the internet. When such a wealth of knowledge source is at the finger tips of many, there are millions still in our country for whom computer technology and internet are still a dream. Therefore one of the best of sharing knowledge with people who do have the technology is to down load relevant materials on relevant subjects and make them available to them in the discs so that those who work among the poor. They will be able to take print outs and translate into local languages and teach those who do not have computer technologies. Hence this booklet contains down loaded article on the subject. It also begin with self generated ideas.

Dr. K. T. Chandy, Agriculture and Environment Education

I. Introduction

This an introduction to both parts I & II. Any human habitat generates plenty of waste water as an outcome of washing, bathing, cleaning of household or animal husbandry complexes, industrial complexes etc. Human habits may vary from single households, villages to mega cities. On an average we can estimate 100 litres of waste water per day per head is discharged into the eco-system. At present all these waste water is being allowed to spread around the villages and cities or allowed to drain into the rivers nearby. Everyone knows how much such a practice is affecting the environment and the health of the people and animals. Hence it has become imperative that we treat all the waste water or sewage and make them ready for safe disposal.

There are many technologies which are employed from ancient times in India as well as in other countries. But as time goes the old technologies give way to new and better technologies most of which are very expensive. The first part of this booklet provides a glimpse of such technologies and the second part will give some designs on low cost technologies that can be implemented at the local level, village level and institutional level. Such designs are only a initiation into the thinking of the need for evolving such low cost technologies since all modern technologies are costly and unaffordable to the ordinary people. The readers are encouraged to evolve similar and better technologies for the waste water management.

The first part of this booklet is a down loaded article from Wikipedia and it is only one of the similar such articles. For the convenience of the users the format of the article in the internet is kept so that anyone who would like refer to the original may be able to do so. Hence even the number of the sub-titles are also kept the same.

Contents

I. Introduction

Part –I

·  1 Origins of sewage

·  2 Process overview

o  2.1 Pre-treatment

§  2.1.1 Screening

§  2.1.2 Sedimentation

o  2.2 Secondary treatment

§  2.2.1 Activated sludge

§  2.2.2 Surface-aerated basins

§  2.2.3 Filter beds (oxidizing beds)

§  2.2.4 Biological aerated filters

§  2.2.5 Membrane bioreactors

§  2.2.6 Secondary sedimentation

§  2.2.7 Rotating biological contactors

o  2.3 Tertiary treatment

§  2.3.1 Filtration

§  2.3.2 Lagooning

§  2.3.3 Constructed wetlands

§  2.3.4 Nutrient removal

§  2.3.4.1 Nitrogen removal

§  2.3.4.2 Phosphorus removal

o  2.4 Disinfection

·  3 Package plants and batch reactors

·  4 Sludge treatment and disposal

o  4.1 Anaerobic digestion

o  4.2 Aerobic digestion

o  4.3 Composting

o  4.4 Sludge disposal

·  5 Treatment in the receiving environment

·  6 Sewage treatment in developing countries

Part – II

I. Treatment of Household Waste Water

II. Multi-chamber Natural Treatment Plant

III. Conclusion

II. Conclusion

Part –I

Sewage treatment, or domestic wastewater treatment, is the process of removing contaminants from wastewater and household sewage, both runoff (effluents) and domestic. It includes physical, chemical, and biological processes to remove physical, chemical and biological contaminants. Its objective is to produce a waste stream (or treated effluent) and a solid waste or sludge suitable for discharge or reuse back into the environment. This material is often inadvertently contaminated with many toxic organic and inorganic compounds.

1. Origins of sewage

Sewage is created by residences, institutions, hospitals and commercial and industrial establishments. Raw influent (sewage) includes household waste liquid from toilets, baths, showers, kitchens, sinks, and so forth that is disposed of via sewers. In many areas, sewage also includes liquid waste from industry and commerce.

The separation and draining of household waste into greywater and blackwater is becoming more common in the developed world, with greywater being permitted to be used for watering plants or recycled for flushing toilets. A lot of sewage also includes some surface water from roofs or hard-standing areas. Municipal wastewater therefore includes residential, commercial, and industrial liquid waste discharges, and may include stormwater runoff. Sewage systems capable of handling stormwater are known as combined systems or combined sewers. Such systems are usually avoided since they complicate and thereby reduce the efficiency of sewage treatment plants owing to their seasonality. The variability in flow also leads to often larger than necessary, and subsequently more expensive, treatment facilities. In addition, heavy storms that contribute more flows than the treatment plant can handle may overwhelm the sewage treatment system, causing a spill or overflow (called a combined sewer overflow, or CSO, in the United States). It is preferable to have a separate storm drain system for stormwater in areas that are developed with sewer systems.

As rainfall runs over the surface of roofs and the ground, it may pick up various contaminants including soil particles and other sediment, heavy metals, organic compounds, animal waste, and oil and grease. Some jurisdictions require stormwater to receive some level of treatment before being discharged directly into waterways. Examples of treatment processes used for stormwater include sedimentation basins, wetlands, buried concrete vaults with various kinds of filters, and vortex separators (to remove coarse solids).

2. Process overview

Sewage can be treated close to where it is created (in septic tanks, biofilters or aerobic treatment systems), or collected and transported via a network of pipes and pump stations to a municipal treatment plant (see sewerage and pipes and infrastructure). Sewage collection and treatment is typically subject to local, state and federal regulations and standards. Industrial sources of wastewater often require specialized treatment processes (see Industrial wastewater treatment).

Conventional sewage treatment involves three stages, called primary, secondary and tertiary treatment. First, the solids are separated from the wastewater stream. Then dissolved biological matter is progressively converted into a solid mass by using indigenous, water-borne micro-organisms. Finally, the biological solids are neutralized then disposed of or re-used, and the treated water may be disinfected chemically or physically (for example by lagoons and microfiltration). The final effluent can be discharged into a stream, river, bay, lagoon or wetland, or it can be used for the irrigation of a golf course, green way or park. If it is sufficiently clean, it can also be used for groundwater recharge or agricultural purposes.

2.1 Pre-treatment

Pre-treatment removes the materials that can be easily collected from the raw wastewater and disposed of. The typical materials that are removed during pre treatment include fats, oils, and greases (also referred to as FOG), sand, gravels and rocks (also referred to as grit), larger settleable solids and floating materials (such as rags and flushed feminine hygiene products). In modern plants serving large populations, sophisticated equipment with remote operation and control are employed whilst in smaller or less modern plants manually cleaned screen may be used.

2.1.1 Screening

The influent sewage water is strained to remove all large objects carried in the sewage stream, such as rags, sticks, tampons, cans, fruit, etc. This is most commonly done with a manual or automated mechanically raked bar screen. The raking action of a mechanical bar screen is typically paced according to the accumulation on the bar screens and/or flow rate. The bar screen is used because large solids can damage or clog the equipment used later in the sewage treatment plant. The large solids can also hinder the biological process. The solids are collected and later disposed in a landfill or incinerated.

Pre treatment also typically includes a sand or grit channel or chamber where the velocity of the incoming wastewater is carefully controlled to allow sand grit and stones to settle, while keeping the majority of the suspended organic material in the water column. This equipment is called a de-gritter or sand catcher. Sand, grit, and stones need to be removed early in the process to avoid damage to pumps and other equipment in the remaining treatment stages. Sometimes there is a sand washer (grit classifier) followed by a conveyor that transports the sand to a container for disposal. The contents from the sand catcher may be fed into the incinerator in a sludge processing plant, but in many cases, the sand and grit is sent to a landfill.

2.1.2 Sedimentation

In the primary sedimentation stage, sewage flows through large tanks, commonly called "primary clarifiers" or "primary sedimentation tanks". The tanks are large enough that sludge can settle and floating material such as grease and oils can rise to the surface and be skimmed off. The main purpose of the primary sedimentation stage is to produce both a generally homogeneous liquid capable of being treated biologically and a sludge that can be separately treated or processed. Primary settling tanks are usually equipped with mechanically driven scrapers that continually drive the collected sludge towards a hopper in the base of the tank from where it can be pumped to further sludge treatment stages.

2.2 Secondary treatment

Secondary treatment is designed to substantially degrade the biological content of the sewage such as are derived from human waste, food waste, soaps and detergent. The majority of municipal plants treat the settled sewage liquor using aerobic biological processes. For this to be effective, the biota require both oxygen and a substrate on which to live. There are a number of ways in which this is done. In all these methods, the bacteria and protozoa consume biodegradable soluble organic contaminants (e.g. sugars, fats, organic short-chain carbon molecules, etc.) and bind much of the less soluble fractions into floc. Secondary treatment systems are classified as

·  fixed-film or

·  suspended-growth.

Fixed-film treatment process including trickling filter and rotating biological contactors where the biomass grows on media and the sewage passes over its surface.

In suspended-growth systems, such as activated sludge, the biomass is well mixed with the sewage and can be operated in a smaller space than fixed-film systems that treat the same amount of water. However, fixed-film systems are more able to cope with drastic changes in the amount of biological material and can provide higher removal rates for organic material and suspended solids than suspended growth systems.

Roughing filters are intended to treat particularly strong or variable organic loads, typically industrial, to allow them to then be treated by conventional secondary treatment processes. Characteristics include typically tall, circular filters filled with open synthetic filter media to which wastewater is applied at a relatively high rate. They are designed to allow high hydraulic loading and a high flow-through of air. On larger installations, air is forced through the media using blowers. The resultant wastewater is usually within the normal range for conventional treatment processes.

A generalized, schematic diagram of an activated sludge process.

2.2.1 Activated sludge

Main article: Activated sludge

In general, activated sludge plants encompass a variety of mechanisms and processes that use dissolved oxygen to promote the growth of biological floc that substantially removes organic material.

The process traps particulate material and can, under ideal conditions, convert ammonia to nitrite and nitrate and ultimately to nitrogen gas, (see also denitrification).

2.2.2 Surface-aerated basins

A Typical Surface-Aerated Basin (using motor-driven floating aerators)

Most biological oxidation processes for treating industrial wastewaters have in common the use of oxygen (or air) and microbial action. Surface-aerated basins achieve 80 to 90% removal of

Biochemical Oxygen Demand with retention times of 1 to 10 days.[2] The basins may range in depth from 1.5 to 5.0 metres and use motor-driven aerators floating on the surface of the wastewater.[2]

In an aerated basin system, the aerators provide two functions: they transfer air into the basins required by the biological oxidation reactions, and they provide the mixing required for dispersing the air and for contacting the reactants (that is, oxygen, wastewater and microbes). Typically, the floating surface aerators are rated to deliver the amount of air equivalent to 1.8 to 2.7kg O2/kW·h. However, they do not provide as good mixing as is normally achieved in activated sludge systems and therefore aerated basins do not achieve the same performance level as activated sludge units.[2]

Biological oxidation processes are sensitive to temperature and, between 0 °C and 40 °C, the rate of biological reactions increase with temperature. Most surface aerated vessels operate at between 4 °C and 32 °C.[2]

2.2.3 Filter beds (oxidizing beds)

Main article: Trickling filter

In older plants and plants receiving more variable loads, trickling filter beds are used where the settled sewage liquor is spread onto the surface of a deep bed made up of coke (carbonized coal), limestone chips or specially fabricated plastic media. Such media must have high surface areas to support the biofilms that form. The liquor is distributed through perforated rotating arms radiating from a central pivot. The distributed liquor trickles through this bed and is collected in drains at the base. These drains also provide a source of air which percolates up through the bed, keeping it aerobic. Biological films of bacteria, protozoa and fungi form on the media’s surfaces and eat or otherwise reduce the organic content. This biofilm is grazed by insect larvae and worms which help maintain an optimal thickness. Overloading of beds increases the thickness of the film leading to clogging of the filter media and ponding on the surface.