Semi-intensive technologies for sewage treatment and storage: intensive versus extensive, construction and operational costs, energy, land requirements, sludge, sanitation, pathogens, refractory pollutants.

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SEMI-INTENSIVE TECHNOLOGIES
FOR SEWAGE TREATMENT

Introducing of a new family of sewage treatment technologies.

EXTENSIVE versus INTENSIVE TECHNOLOGIES.

Extensive technologies.

Extensive reactors include stabilization ponds, constructed wetlands, wastewater reservoirs and others. They are large and the residence time of the effluents within the reactor is not measured in hours but in weeks or months. The concentration of microbial biomass is low and oxygen is supplied by the photosynthetic activity of algae and by atmosphere reaeration. Several complex low-rate processes and reactions occur within these reactors. Biological break-down is just one of the several and complex low-rate reactions occurring within the reactor. Thus, compounds which are toxic to microbial biomass are also removed.

Low-rate reactions such as break-down of hard detergents and pesticides become relevant due to the long retention time. Pathogens are removed by singlet oxygen formed through photochemical activation of dissolved organic matter, high pH related to algal photosynthetic activity and slow settling.

The main disadvantage of extensive technologies is the large extension of land required to achieve long residence times (several weeks) and assure proper oxygen supply by photosynthesis and diffusion. If the reactors are constructed smaller or very deep in order to reduce land requirements the whole system may become anaerobic.

Extensive reactors release no or very little sludge.

Intensive technologies.

Intensive reactors such as activated sludge are designed to accelerate the degradation of organic matter. Sewage is put in close contact with a concentrated biomass of microorganisms and a large amount of dissolved oxygen is mechanically supplied to the reactor. Effluents leaving the reactor pass through a sedimentation basin (the clarifier) where the biomass settles.

Intensive reactors have excellent performance in the removal of organic matter, doing this in reduced space and short time (the residence time of the effluents within the reactor is short: e.g., 8 to 14 hours in activated sludge systems).

Refractory compounds with low degradation rates (hard COD) are poorly removed in intensive reactors. The addition of post-treatment stages to the system (activated carbon, flocculation, filtration, etc.) involves high investment and operational costs, high energy consumption, and release of a series of sludges whose treatment and disposal is also difficult and expensive.

Pathogens are also poorly removed in intensive reactors. This problem can not be easily overcome by chlorination, since chlorination of wastewater effluents presents several shortcomings.

Besides, intensive reactors lack buffer capacity due to the short residence time making the system sensitive to transient quality and quantity interruptions and thus reducing their reliability.

Intensive reactors have no storage capacity.

Intensive reactors release large amounts of sludge whose treatment and disposal is difficult and expensive.

Comparison of extensive versus intensive reactors for sewage treatment

THE FAMILY OF THE SEMI-INTENSIVE TECHNOLOGIES.

Semi-intensive technologies represent an effort to combine the best characteristics of extensive and intensive technologies.

The basic idea is to use intensive units (retention time of 6-12 hours) or semi-intensive units (retention time of 2-4 days) for the partial removal of organic matter, and then complete the treatment in extensive units which can be designed smaller and with less land requirements since part of the organic loading was already removed in the previous steps.

There is a wide spectrum of semi-intensive technologies, from the most extensive ones such as anaerobic ponds followed by facultative ponds and wastewater reservoirs, to the most intensive ones such as stand alone aerated batch-fed wastewater reservoirs.

Main advantages of semi-intensive technologies are:

Require less energy than intensive ones
Require less land than extensive ones
Remove as much BOD as intensive ones
Remove as much pathogens and refractory pollutants as extensive ones
Release no or very little sludge

COSTS

The cost of a sewage treatment plant depends on numerous parameters and then varies a lot from project to project. The following table presents just a 'rule of thumb'.

Item	Intensive technologies	Semi-intensive technologies	Extensive technologies
Construction costs US$ per capita (land not included)	$ 100 -120	$ 50 – 70	$ 20 – 30
Land use	little	medium	much
Operation and maintenance costs	high	medium	low
Simplicity of operation	complicated	simple	very simple

Some examples of lay-outs applying semi-intensive technologies.
Aerated lagoons are single-cell low-energy (~ 6 W/m3).