INTRODUCTION

The need for the removal of odour form sewage plants is becoming more topical worldwide. Sewage plants were built just outside the existing developed areas so that they were close to the source, but outside the main populated areas.

As towns and cities have grown, the new developments are now close to the old sewage plants. Odours generated do cause major concern to the people living in the area, and odour removal plants are required to satisfy the public.

In other areas, the odours generated are so bad that development is not possible. Odour removal allows certain areas to be developed, often bringing in far more income to the authorities than the cost of purchasing and operating an odour removal plant.

TECHNOLOGY BASIS AND EXPERIENCE

As the main water companies in the UK privatized, the pressure mounted quite quickly on them to remove the odours generated by the various plants.

The UK consultant to FAMSYSTEMS has been involved in building numerous plants with great success. The list of references has been extended over the last few years, to represent one of the most complete spectrum of plants possible. Based on experiences gained, various technologies can be offered:

      - Chemical systems with or without catalyst regeneration
      - Biofilters
      - Bioscrubbers
      - Activated carbon systems

Each process has its own advantages. It is difficult to specify clearly which technology is better for specific duties. The best policy is to look at the duties of the plant and where possible evaluate the operating efficiency and compare this to the overall cost of the system, including capital and operating costs.

It should be noted that the odours generated in a sewage plant are based on hydrogen sulphide and mercaptans. These chemicals have very low Odour Threshold levels. Typical value for H2S is 0.5 parts per billion.

This fact makes the design of odour removal plants of utmost importance. If the design is not done correctly the required efficiency is not obtained, the smell will not be removed resulting in continued complaints from the public.

It is for this reason that the experiences gained over the years are invaluable to ensure that the odour removal plant removes all traces of smells.

PROCESS TECHNOLOGIES

As indicated above, various technologies exist, and we describe each system to introduce the process and give some guidelines as to the possible applications.

1. CHEMICAL SCRUBBERS

Vertical Scrubbers

These are standard packed towers where the gases flow vertically upwards and are contacted with a scrubbing liquor. The scrubbing liquid is sprayed onto a packed section so that it is well distributed to give maximum contact area.

The scrubber diameter is determined by the gas flow and the height is determined by the efficiency required. Scrubbers are manufactured from Polypropylene reinforced with fibreglass. The PP is fully resistant to all liquid and gas concentrations so that it offers a long-term solution. The fibreglass reinforcing is designed to give the plant the mechanical strength required as a freestanding unit capable of being installed outside.

The chemical scrubbers use a mixture of caustic soda and hypochlorite to remove all hydrogen sulphide and mercaptans present in the gas.

The controls are simple and no special computer control is required. The systems are supplied complete with dosing systems for the chemicals to ensure the overall process is stable and balanced.

The effluent produced is a stable sulphate solution, which can normally be discharged, as the salt content is low.

The efficiency of the scrubbers can be designed to any value and it is suitable for very high inlet concentrations of odours. Efficiencies in excess of 99.99% can be obtained.

Cross flow wet scrubber

The cross flow scrubber design is a horizontal scrubber, which can be pre-manufactured in the workshop to offer a compact and inexpensive scrubber solution for odour removal. The unit allows the designer to select both height and width to suit the space available.

The unit comprises of a bed of random tower packing irrigated by the scrubbing liquid from the front and the top of the bed to ensure complete and even wetting of the packing.

The scrubbing liquid is circulated from an internal sump and controls are similar to the vertical units.

The contaminated air flows horizontally through the bed, where absorption takes place. After the packed bed a special mist eliminator ensures that no droplets are carried with the gas past the scrubber.

As usual mass transfer calculations are done to ensure an optimum economic design.  As for the vertical scrubbers, the plant is offered complete with controls, dosing systems etc.

The cross flow scrubber is suitable for odour removal and general chemical absorption with removal efficiencies in the range 95 to 99.99%.

2. BIOFILTERS

Biofilters are designed to adsorb odours from sewage plants and industrial installations. The units comprise of a shallow bed of specifically selected organic fibres or woodchips though which the odours are passed. The gas flows though the packing vertically upwards..

The units operate by absorbing the odorous compounds onto the surface of the fibres. These are then broken down biologically into mainly water and carbon dioxide by micro-organisms attached to the fibres.

The fibres are selected to allow free passage of the air and also to minimise the frequency of replacement over time. Automatic water sprays maintain the required moisture level in the bed and also remove the degraded products.

Biofilters do not require any chemicals, which reduces their overall operating costs. They are however much larger than chemical scrubbers and space requirements are often a consideration.

They are suitable for levels of contamination up to 30ppm of hydrogen sulphide. It is also important that the H2S levels are fairly constant or at least vary slowly over time. Removal efficiencies are lower than chemical units and are limited to 90 – 98%.

The biofilter offers many advantages, but due to its operating principle there are limitations in its use.

3. BIO-SCRUBBERS

Bioscrubbers are similar in design to the vertical scrubbers described above. Again the unit is packed with random packing.

The air passes vertically upwards and the scrubbing liquor, which is the treatment works final effluent, is recirculated to fall downwards over the packing contacting the gas. The liquid is seeded with a bacteriological culture. The resulting microbe population on the packing feeds on the odours and is sustained by a constant feed of final effluent. The resulting bleed from the unit is passed back to the inlet works.

It can be seen from the description that this unit, although fully biological, does offer a higher efficiency than normal biofilters.

They are suited for inlet concentration of up to 300ppm of H2S. However as was the case with biofilters, the inlet odour levels must be constant or increase very slowly to enable the microbe population on the packing to grow to suit.

If there are sudden increases in the inlet levels it is an alternative to install an activated carbon unit behind the bioscrubber.  The activated carbon unit (described below) would then only be used to trim the peaks, which the scrubber cannot handle.

At the same time the activated carbon life is increased as the majority of the work is done by the bioscrubber.

Efficiencies of the bioscrubber are again in the region of 90 - 99%

4. ACTIVATED CARBON

This was the original process used for the removal of odours. Whereas it is no longer used on large units due to the problem of replacement cost as well as danger of fire, activated carbon has still applications and we have used it for a number of special applications.

The carbon absorbers consist of a fixed bed of random pellets through which the contaminated air flows. The high surface area of the carbon allows very efficient removal of the contaminants. Typical figures of 1250m2 per gram are typical.

The carbon is specially selected with coal or coconut shell based carbon being preferred. For certain applications chemically impregnated carbons are used to enhance he efficiency and stability of the system. Carbon can be regenerated, but this is only economical for larger units.

In general one must assume that the carbon has a limited life and requires replacement after a certain time period. This makes the carbon absorbers more expensive to maintain that the biosystem described above.

The main advantage of activated carbon is that within the time limitation of the material, removal efficiency of 100% can be obtained.

PLANT CHOICE

It is difficult to confirm the best choice of odour removal system in general terms. The following aspects will be looked at by us to formulate alternatives:

  • The inlet concentration of H2S
  • The gas volume
  • The variation in odour concentration as well as the rate of variation
  • The efficiency required.
  • The space available

It is also important for the client to specify the required outlet levels. With the low Odour Threshold level one can specify as follows:

  • plant to produce air at below the Odour Threshold level of 0.5ppb
  • the plant position on the site is pinpointed and normal and maximum winds are specified to allow us to use a dispersion calculation to ensure that the OTV is maintained at the fence of the plant.

Obviously the choice of cleaning efficiency required will determine the overall cost and choice of the plant.

SERVICES OFFERED

To offer this type of technology to the South African industry Famsystems has put together a system so that full turnkey contracts can be offered at reasonable costs.

  • Process design is done by UK consultant based on his vast experience in the field.s
  • Mechanical design is done in South Africa, minimising costs. Famsystems uses AutoCAD 14 2000 for the generation of drawings.
  • Project control is done using Microsoft Project.
  • Famsystems has its own chemical engineers to control the project from the beginning to commissioning.
  • As all of the above structures are manufactured from plastics and fibreglass, the extensive specialised manufacturing facilities of FAMSYSTEMS are used. Manufacturing is controlled by experienced staff and full Quality Control measures are set up to ensure that the Quality produced is in line with the requirements of the project.
  • FAMSYSTEMS uses normally their own range of Tecnium plastic pumps for the circulation of the scrubbing liquor. These pumps use PP as the main material of construction to ensure full corrosion resistance to the liquids.  Using own manufacturing facilities and equipment such as pumps, reduces the overall cost of the plant significantly.
  • Electrics and civils are subcontracted to suitable contractors usually based in the vicinity of the plant. If preferred the client can supply these services themselves and FAMSYSTEMS will provide specifications and drawings so that this work can be completed without any problem.
  • Installation of the plant is done under control of FAMSYSTEMS's own team. All plastic piping and associated equipment is installed by specialists in the field. As FAMSYSTEMS has a limited steel fabrication facility, simple supports are also manufactured in the factory.
  • Commissioning is done by a FAMSYSTEMS engineer.

    • FAMSYSTEMS does not offer the actual measurement of the inlet or outlet concentration of odours as it is felt that this should be done by an independent third party. Names of accredited companies can however be supplied

Copyright © Famsystems (Pty) Ltd. 2002