Sewage Treatment Plants

Sewage treatment is the process of removing contaminants from wastewater, primarily from household sewage. It includes physical, chemical and biological processes to remove these contaminants and produce environmentally safer treated wastewater (or treated effluent).

A by-product of sewage treatment is usually a semi-solid waste or slurry, called sewage sludge, which has to undergo further treatment before being suitable for disposal or land application.

Sewage treatment may also be referred to as wastewater treatment, although the latter is a broader term which can also be applied to purely industrial wastewater. For most cities, the sewer system will also carry a proportion of industrial effluent to the sewage treatment plant which has usually received pretreatment at the factories themselves, to reduce the pollutant load.

If the sewer system is a combined sewer, then it will also carry urban runoff (storm water) to the sewage treatment plant. Sewage water can travel towards treatment plants via piping and in a flow – aided by gravity and pumps. The first part of filtration of sewage typically includes a bar screen to filter solids and large objects, which are then collected in dumpsters and disposed of in landfills. Fat and grease also are removed before the primary treatment of sewage.

Sewage is a mixture of domestic and industrial wastes. It is more than 99% water, but the remainder contains some ions, suspended solids and harmful bacteria that must be removed before the water is released into the sea.

The treatment of wastewater is divided into three phases: pretreatment, primary treatment and secondary treatment.

Pre-treatment

Large solids (with a diameter of more than 2 cm) and grit (heavy solids) are removed by screening. These are disposed of in landfills.

Primary Treatment

The water is left to stand, so that solids can sink to the bottom, and oil and grease can rise to the surface. The solids are scraped off the bottom, and the scum is washed off with water jets. These substances are combined to form sludge

Secondary Treatment

The sludge is further treated in ‘sludge digesters’ –large heated tanks in which its chemical decomposition is catalyzed by micro-organisms. The sludge is largely converted to ‘biogas’, a mixture of CH4 and CO2, which is used to generate electricity for the plant. The liquid is treated by bacteria which break down the organic matter remaining in solution. It is then sent to oxidation ponds where heterotrophic bacteria continue the breakdown of the organics and solar UV light destroys the harmful bacteria.

The role of the laboratory

A wide variety of analytical tests are used to determine the purity of the wastewater at various stages of treatment, so that the possibility of harm to either people or the environment is minimized.

Sewage Treatment Technologies

MBBR (Moving Bed Bio Reactor) TECHNOLOGY

Moving Bed Bio Reactor Systems consist of activated sludge aeration systems, where the BOD-COD consuming bacteria feed and multiply on the inner and outer surface of the inert media. This media acts as a large surface area for optimal contact between the bacteria, oxygen and influent water. With sufficient supply of oxygen, the bacteria culture breaks down the organic compounds in the influent, thus resulting in substantial reduction in the BOD & COD.

Working Process

  1. The media in the aeration tank are kept in suspension at all times. This carrier act as breeding and feeding ground for the bacteria to breakdown the influent BOD & COD.
  2. These bacteria need a large amount of oxygen to sustain their growth, and cannot sustain themselves only on the dissolved oxygen. Therefore, excess oxygen is supplied by means of an Air Blower, and dispersed by Air Diffusers placed at bottom of the tank.
  3. Over time, as the bacteria consumes the BOD and COD and multiply, they get detached from carrier media, and can be removed in the form of sludge
  4. The removed sludge containing moisture is passed through the filter press separating the sludge and moisture. Separated moisture, in form of filtered water, is recycled back to the head of works, and the sludge is sent for disposal by incineration or landfill.

Advantages

  1. Low space requirement
  2. Cost-effectiveness
  3. Low maintenance cost
  4. Longer service life
 

SBR (Sequential Batch Reactor) TECHNOLOGY

SBR System consists of a reactor constructed from a water-tight material such as concrete or mild steel (with corrosion-resistant coating). It carries out operations such as Filling, Reacting, Settling and Decanting, in single or multiple reactor tanks operating in a series. Initial anaerobic/ anoxic condition followed by aerated condition of the influent water results in better output quality of product water.

Working Process

  1. Treatment procedure starts with the Filling process, during which, the influent is added to the leftover Biomass from the previous batch. Due to the absence of oxygen, anoxic/ anaerobic conditions created during this process results in Nitrification and De-nitrification, and considerable BOD and COD reduction.
  2. Once the above process is completed, influent is given vigorous aeration, so that the aerobic bacteria feed on the remaining organic matter. The aeration cycle continues till the desired effluent loads are reached.
  3. Once the aeration is stopped, the mixed liquor is allowed to settle. After settling, the decanter system draws out the clear treated water, and the system cycle is repeated again.

Advantages

  1. SBR System enables efficient and higher reduction of carbonaceous matter from the influent.
  2. Lower energy consumption, as the system works in anaerobic and aerobic conditions.
  3. Consistent and pure outlet quality,owing to better reaction/ degradation of biomass.
  4. Lesser sludge production compared to conventional treatment systems.

Solido Treatment System

  • This small sewage water treatment system works as a Sequencing BatchReactor.
  • It has two treatment stages: primary treatment, and the SBR phase.
  • During primary treatment, floating inorganic pollutants and pollutants that sediment are separated and held back from entering the wastewater stream. Only dissolved or floating pollutants reach the SBR.
  • The biological treatment of one cycle in the SBR lasts 6 hours. It is made up of 4.5 to 5 hours of aeration and 1 to 1.5 hours of sedimentation.
  • The excess sludge that is left over from the biological treatment, is pumped into the primary treatment, and accumulated there.
  • The treatment cycle is controlled by a control unit. This means the system can be adapted to the local conditions, optimizing the operating values.

DAF (Dissolved Air Flotation) TECHNOLOGY

In this technology, the feed water to the DAF float tank is often (but not always) dosed with a coagulant (such as Ferric Chloride or Aluminum Sulfate) to coagulate the colloidal particles, and/ or a flocculant to conglomerate the particles into bigger clusters.

A portion of the clarified effluent water leaving the DAF tank is pumped into a small pressure vessel (called the air drum), into which, compressed air also is introduced. This results in saturating the pressurized effluent water with air.

The air-saturated water stream is recycled to the front of the float tank,which flows through a pressure reduction valve just as it enters the front of the float tank, resulting in the air being released in the form of tiny bubbles. Bubbles form at nucleation sites on the surface of the suspended particles, adhering to the particles. As more bubbles form, the lift from the bubbles eventually overcomes the force of gravity. This causes the suspended matter to float to the surface, where it forms a froth layer, which is then removed by a skimmer. The froth-free water exits the float tank as the clarified effluent from the DAF.

Advantages

  1. Low capital investment and operational costs
  2. Continual self-cleaning
  3. Minimal maintenance
  4. Stable operations
  5. High-quality clarified water
  6. Simple installation

MBR (Membrane Bio Reactor) TECHNOLOGY

The MBR System consists of a submerged hollow tube.A PVDF micro-porous membrane is used to separate the treated water from the suspended growth-activated sludge process, resulting in purified water recovery. MBR Membranes have a pore size of ~0.2µm. Thus, most of the BOD, COD, TSS,etc of the influent are separated from the product stream.

Process

  1. Skid-mounted MBR Modules are submerged in the activated sludge aeration tank.
  2. In presence of sufficient Oxygen, the aerobic bacteria in the activated sludge process feed and multiply on the influent BOD and COD, converting them into sludge.
  3. Partial vacuum created by the vacuum pump attached at the outlet of MBR module forces the treated water to travel outside-in through the membrane wall separating the sludge from treated water.
  4. Since all the solids are retained within the tank, this process reduces the requirement of MLSS ratio maintenance for continuous activated sludge process.

Advantages

  1. Eliminates the need for Clarifiers, thus require smaller plant foot print.
  2. Small pore size of MRB membranes result in MF/UF quality treated water.
  3. Eliminates the need for MLSS ratio maintenance, thus reducing frequency of aeration system failure.
  4. Can be easily integrated into the existing system.
  5. PVDF material enables in longer service life of membranes.
  6. Less sludge generation.

Trickling Filters

trickling filter, also called trickling biofilter /biofilter / biological filter / biological trickling filter, is a fixed-bed biological reactor that operates (mostly) under aerobic conditions. Pre-settled wastewater is continuously ‘trickled’ or sprayed over the filter. As the water migrates through the pores of the filter, organics are aerobically degraded by the biofilm covering the filter material.

Introduction

Trickling filters are conventional aerobic biological wastewater treatment units, such as Active Sludge Systems or Rotating Biological Contactors. The advantage of all these systems is that they are compact. However, they are high-tech, and generally require skilled staff for construction and operation.

Trickling filters are a secondary treatment after a primary setting process (see also ‘septic tanks’ or ‘pre-treatment’).

The trickling filter consists of a cylindrical tank, and is filled with a highly specific surface areamaterial such as: rocks, gravel, shredded PVC bottles or special pre-formed plastic filter media. A highly specific surface provides a large area for biofilm formation. Organisms that grow in the thin biofilm over the surface of the media, oxidize the organic load in the wastewater to carbon dioxide and water, while generating new biomass. This happens mainly in the outer part of the slime layer, which is generally of 0.1 to 0.2 mm thickness.

The incoming pre-treated wastewater is ‘trickled’ over the filter with the use of a rotating sprinkler, prompting the filter media to go through cycles of being dosed and exposed to air.

However, oxygen is depleted within the biomass, and the inner layers may be anoxic or anaerobic.

The word filter is somehow misleading, as physical straining of solids is only marginal. The removal of organic substances occurs from bacterial action. Therefore, trickling filters are also called bios or biological filters, to emphasize the filtration aspect. Fixed film biological treatments  also are used in other common treatment processes such as Rotating Biological Contactors of Fixed Film Activated Sludge Systems.

Advantages

  • Can be operated at a range of organic and hydraulic loading rates
  • Resistant to shock loadings
  • Efficient nitrification (Ammonium Oxidation)
  • In combination with primary and tertiary treatment, offers higher effluent quality compared to BOD and Suspended Solids Removal. Higher efficiency in terms of pathogens too.
  • Small land area requirement compared to constructed wetlands.