Sequencing Batch Reactor SBR

Overview

Sequencing Batch Reactor SBR in wastewater treatment plant

Category / type: suspended biomass, activated sludge, discontinuous flow or sequential process.
Application: hotels, camping, resorts; tourist villages.
Population Equivalents (PE) served: from 2 up to 50 (Packaged and/or site assembled) and more than 50 tailor-made.
Normative references and guarantees: EN 12566-3, d.o.p. According to Regulation (UE) n.305/2011.
Material: concrete, polyethylene, polypropylene, glass-reinforced plastic (GRP).


Sequencing batch reactor (SBR) is a suspended biomass activated sludge plant. It works with discontinues flow.
Actually it is the first type of activated sludge system designed in the early ‘900.
Gradually replaced by constant flow plants for technical reasons, it is recently regaining interest in/for small communities applications.
SBR solution requires less and more simple components then conventional activated sludge plants. There’s no secondary sedimentation basin and no sludge recirculation pumps because all treatment phases take place in the same volume, especially nitrification and denitrification processes. Treatment plant results very effective because each phase use entire volume and sedimentation process occurs in ideal and perfect static conditions.
Depending on required quality results, there are different operation schemes.
The biological process is the same as the conventional plants: microorganisms decompose organic matter and nutrients, content in wastewater, and produce a good quality effluent. In an activated sludge plant, treatment usually consists on five stages: filling, reaction, settling, extraction and idle (excess sludge disposal when necessary). In a SBR treatment plant processes are realised in a single volume setting a proper timeframe for different steps. Oxygen is supplied by an aeration system and bacteria oxidise organic matter as a conventional activate sludge plant but in only a part of the treatment process.
Later aeration supply stops to allow sludge to settle (for times between 1 and 5 hours). After sedimentation time clarified water is discharged and sludge remain stored on bottom of the basin.
Excess sludge is removed by pumps or air-lift during the idle phase.
After the idle phase the volume is filled again with waste water and a new cycle begins.
Two parallel lines are recommended for medium-sized plants in order to treat a continuous flow and to guarantees a great flexibility during maintenance operation.
For small plants (for example small residential areas) a single reactor is often used. Inflow is stored in a primary sedimentation tank working as an equalisation vessel.
SBR plants are also suitable for small solution (starting from few equivalent inhabitants).
In a SBR reactors an anoxic, nitrification and denitrification phases are also present.
In order to remove solids and scums that can block pipes, pumps, and valves mechanical it’s necessary to install pretreatments as fine screening, grease trap (oil and grease removal) or septic tank.
For very small systems a further simplification is possible: pre treatments may be removed, but not on domestic grey water (Grease trap).
Effluent can be discharged into a receiving body or subjected to further treatment according to the standard purification required (tertiary treatment) or stored and reuse after a disinfection step (see water recycling).
To reduce and optimize personnel operations a significant automatization is possible
Electronics and microprocessors are a significant support for the automation (monitoring and alarms). Even in this case, an additional storage tank for excess sludge allows to reduce the cleaning operation frequency; therefore it represents a considerable savings (considering the whole plant life).

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How It Works

SBR systems are usually used for small plants. They operate in extend aeration field and so nitrification of nitrogen compounds is always guaranteed.
Operating cycle must set depending on main pollutants (BOD, NO2, NO3, P etc.) concentration required in the effluent.
There are different guidelines and design approach to plan a SBR plants, a great number of SBR type are available on the market.
Sizing must be based on empirical experience in biological treatment and on tested and certified performance for each plant.
Some authors indicate typical design values as load factors from 0.10 to 0.15 kg BOD5 / kg SS per day and a biomass concentration from 2.5 to 3.5 kg SS / m3.
The most frequently changed parameters to design a batch reactor are: number of days cycles, phases duration, feeding mode.
For example, the Environmental Protection Agency (EPA, 1999), for civil wastewater suggests to consider 6 cycles per day, each lasting 4 hours, an organic load factor (F / M) from 0.15 to 4.0 kg BOD / kg SS per day and an hydraulic residence time from 6 to 14 hours.
For industrial wastewater parameter range is larger and it needs a focused study for each specific case.
The main parameter for civil wastewater plant (or equivalent) projects is the population equivalent (PE); normally it refers to 60 g BOD5 per day and a flow rate from 150 to 250 litre per day.
Design requires calculation of the nominal daily organic load in terms of BOD 5 and the nominal hydraulic flow (QN) in m3, according to EN 12566-3 and technical procedures.
Population equivalent (PE or sometimes used p.e.) is a conversion value which aims at evaluating non-domestic pollution in reference to domestic pollution fixed by EEC directive (Council Directive 91/271/EEC concerning Urban Waste Water Treatment) at 60 g/day related to BOD5.
European legislation defines Population total (PT) as a sum of population and population equivalent (PE)
A proper wastewater characterization for incoming organic loads a better estimation of nutrients are possible considering the possibility of dosing additives for biological reactions.
Tertiary treatment can added downstream to increase effluent quality or to recycle water (disinfection or filtration step to be added).
Sizing should be more accurate then primary treatment; biological processes require more constant water supply (therefore organic loads). Equalisation tank is always a good solution to ensure a constant flow and a pollutant load.

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Operation and maintenance

Qualified personnel is required for maintenance operations, as for all wastewater treatment plants. Especially cleaning operation must be carried out by competent and authorized personnel.
Mechanical equipment (mixers, aerators and pumps) must be subject to periodic inspections and maintenance.
It is necessary to ensure a regular oxygen supply in oxidation compartment and dissolved oxygen rate must be checked by DO (dissolved oxygen meter). Sludge, inflow and outflow, are checked by specific instruments as TSS probe, pH meters, thermometer, Imhoff cone, etc.
Inlet water is checked to evaluate treatment performance and to control abnormal intakes of pollutants that could completely stop biological process or allow development of an useless bacterial population (for example, filamentous bacteria).
Nutrient and chemicals may be added to control bacteria process and to improve treatment efficiency.
These operations can be realized with a system partly or wholly automatized equipped also by remote control.
Programmable Logic Controller (PLC) is very useful on wastewater plats. It allows to connect all controls and electro-mechanical equipment with a personal computer. Plant management became very quick and smart.
SBR plant can work with discontinuous flow but can’t afford long inactivity time, in this case a new start up is needed.

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Regulatory framework

A sequencing batch reactor is a wastewater treatment plant and for this reason, small installations up to 50 total population refer to the EN 12566-3 specifications.
The European standard EN 12566-3 “specifies requirements, test methods, the marking and factory production control for packaged and/or site assembled domestic wastewater treatment plants (including guest houses and businesses) used for populations up to 50 inhabitants…”
The manufacturer shall state the nominal organic daily load, expressed in kg of BOD5 or BOD7 per day and the nominal hydraulic daily flow, expressed in cubic meters per day.
Plant’s Nominal Designation is the proper daily organic load value (QN) (3.7).
The small wastewater treatment plant must demonstrate compliance with the performance of efficiency… and to operational data declared by the manufacturer: the declaration must be expressed in percentage.
It is also to be considered that the structural resistance and the hydraulic efficiency must be tested only on a representative model of the product line. Representative model for structural resistance test must be the bigger, supposedly as the one of series with the worst characteristic, while the one for hydraulic efficiency test must be the smaller, presuming the one with the worst efficiency.
The standard describes statics and hydraulics methods to determinate resistance and hydraulic efficiency.
Informations to be included in the test report and in the CE mark are also detailed in ZA.3 annex.
For small domestic wastewater treatment plants up to 50 PT (therefore for activated sludge plants), a Declaration of Performance (DoP), is required, as provided by the EU Regulation. 305/2011.

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Features and benefits

Sequencing batch reactor is a plant for small-medium population equivalent.
It needs qualified personnel, care in connection operation and technical equipment (spare parts, monitoring equipment, etc.).
Activated sludge process is appropriate in almost all climatic conditions, although treatment efficiency is very responsive to temperature. Low temperatures require higher volume to get the same quality result. ASP
SBR are used to treat domestic or equivalent wastewater from small and medium-sized civilian residential areas, for hotels, motels, campings sites, holiday village, etc.

Advantages (benefits)

  • resistant to organic and hydraulic shock;
  • optima sedimentation (null horizontal velocity);
  • good BOD and pathogens reduction;
  • good nutrient and suspended solid (SS) reduction;
  • good effluent quality;
  • bigger area compared to activated sludge plant;
  • wastewater homogenization;
  • system integration (tertiary, irrigation reuse, etc.);
  • automatic and remote controls function;
  • reduced and stable excess sludge.

Disadvantages

  • higher initial investment compared to primary treatments;
  • power supply needed;
  • higher operating costs and energy consumption;
  • multiple line or storage tank for constant flow rate;
  • skilled personnel needed;
  • system start up needed;
  • spare parts and planned maintenance;
  • accurate sizing than simple primary treatments;
  • excess sludge disposal.

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Products in category Sequencing Batch Reactor SBR

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