Efficiency by Design and Commitment by Heart

Since 1986

1. Operating Instructions

Use of Products

Use OTT products only in accordance with applicable OTT instructions and specifications.

Receiving Inspection

Check each delivery immediately upon receipt for:

  • transport damage
  • correct quantities
  • correct products delivered.

In the event of transport damage, immediately inform the forwarding agent and OTT.

Refuse acceptance in the event packaging cartons or products delivered are not completely intact.


Transport all OTT products at the jobsite carefully and in their original packaging, taking care not to damage them. Do not drop, abruptly set down or tip over packaging.

Do not repackage OTT products.


Store OTT products in compliance with DIN 7716 in dry and well ventilated areas and only in their original packaging. Do not expose them to high temperatures, dust, below-freezing temperatures or continuous sunlight. Do not store them in a way which causes them to be pulled, compressed or deformed in any way. Depending on storage conditions, storage of membranes/diffusers may cause temporary closure of perforation, resulting in changes in the membranes'/diffusers' pressure drop and bubble pattern. We therefore do not recommend storage of membranes/diffusers.

If you will not be able to install and use all membranes/diffusers immediately after receipt, please consult OTT in advance. We can supply your products in partial deliveries according to your needs, i.e. in smaller quantities permitting installation and start-up without delay.


Prerequisites for Installation

Immediate Installation

Do not install diffusers/membranes until all construction measures and other preparatory work have been successfully completed. Install diffusers/membranes only if you will be able start up the aeration system as specified in Section 1.9. immediately after their installation.

Requirements for Air Header System

The air headers must be secured against buoyant uplift and other displacement. In addition, a stress-relieved installation has to be guaranteed.

The permissible tolerances for installation of rectangular crosssection headers are given in EN 10219-2. The permissible tolerances for inst lation of round cross-section headers are given in EN ISO 1127 T3 / D3.

Requirements for Supply Openings in Air Header System

Drilled holes for air supply from headers must be burr-free. They must not be located in the vicinity of welding seams and have to be arranged absolutely centrally. Also observe our specifications concerning drilled holes for the specific diffuser and header types given in Section 3 (colour code: green).

Application for MAGNUM® diffusers:
Application for STANDARD diffusers:

Levelling of Headers

The air headers must be installed with adjustment capability providing levelling of all headers in the basin to within a tolerance of +/- 1 cm / inch.

Levelling of the installed diffusers can be checked by filling the basin with clean water to the upper edge of the diffusers.

Cleaning of Headers and Basin Prior to Installation

Before installing OTT products, clean all laterals and other upstream air piping to remove all foreign matter such as dust, metal shavings, sand, etc. All such piping must be completely dry.

Remove any loose matter, objects and/or debris present in the basin and use a broom to sweep its surface completely clean.

Transfer of OTT Products Into Basin

Always transport diffusers with care and in their original packaging.

Take care not to damage the products when transferring.

Ideally the cartons or pallets containing the products should be transferred unpacked into the basin to avoid damage in handling.

Installation Instructions

Detailed installation instructions for the individual OTT products are given in Section 2 (colour code: light blue).

Install OTT products only in accordance with these instructions and only by qualified personnel observing all further specifications given in this handbook.

Configuration of Diffusers in Basin

The number, configuration and clearance of the diffusers must be in conformance with the layout drawings for your basin.

Filling of Basin After Installation

When filling the basin with water or wastewater after installation, take care to prevent damage to diffusers and membranes due to forceful flow impact.


Leak Testing

Immediately after completing installation of the aeration system, carry out a test run in the basin under clean water:

  1. Fill the basin with clean water up to a level 10 cm / 4" above the diffusers. During filling the basin the aeration may be activated yet. Thus you will avoid intrusion of water into the system in case of leakage.
  2. As soon as the diffusers are covered with approx.10 cm / 4" of water, stop air infeed one minute later.

Any leaks in the system will be indicated by air bubbles from the leaking spot. Repair all leaks. Any leaks in the system will be indicated by air bubbles from the leaking spot. Repair all leaks.

In the event a leak is found in a diffuser/header interface:
First check that the mounting bolt is tightened to the correct torque and check that the diffuser is properly seated on the header. If this does not resolve the leakage, check the seal located between the diffuser and the header and replace if necessary. If this still does not resolve the leakage, replace the diffuser.

In the event a leak is found in a membrane:
Replace the membrane, taking care to use correct procedures (given in Section 2.5).

In the event a leak is found in the piping system:
Check correct installation and fit of the pipe section. Repare or replace damaged pipe sections. 

When doing clean water tests, be certain that the clean water used does not contain excessively high concentrations of calcium and iron. These can precipitate and form incrustations on membranes.

Bubble Pattern Test

The bubble pattern test is a visual check of the air output distribution from the diffusers. Carry out the bubble pattern test directly after the leak test, using an air throughput through the diffusers of at least 2 Nm3/h (1.25 SCFM) per m of membrane length. If a non-uniform bubble pattern is observed in the test, run in the diffusers in accordance with Section 1.9.4. Membranes with poor or uneven air throughput may in addition require "massaging" by hand to activate them, i.e. to open any closed or partially closed perforation slits. Then repeat the bubble pattern test. If the bubble pattern is still not uniform, replace any membranes having no air output or impaired air output.


Filling of Basin

After Diffuser/Membrane Installation When filling the basin with water or wastewater after installation, take care to prevent damage to diffusers and membranes due to forceful flow impact. To prevent water from entering the aeration system in the event of leaks, run the maximum permissible air throughput (see Section 1.9.2 / 1.9.3) through the diffusers while filling.

Running-In of Diffusers

Immediately before start-up, run in the diffusers over a 24 hour period at maximum air throughput (given in Section 1.9.2).

Make certain that all diffusers are run in with the same air throughput and over the same time period.

Running-In of Diffusers Before Oxygen Input Test

Conducting oxygen input testing only after having operated or run in the diffusers for a period of 48 hours with maximum air throughput (given in Section 1.9.2). The oxygen input test must also comply in all other aspects with EN 12255-15 in its latest version. Make certain that all diffusers are run in with the same air throughput and over the same time period.

Procedures in the Event of Delayed Start-Up

If operation of the aeration system is not started immediately after running-in or oxygen transfer rate testing, this may have a negative effect on its performance. Typical problems encountered here include:

  • Physical damage due to work procedures in the basin or objects falling into the basin
  • Algae formation (if basin is filled), both in the water itself and resultant algae depositing on membranes
  • UV ageing of EPDM membranes due to sunlight exposure
  • Change in pressure drop and the air output behaviour of the membranes
  • Ice formation (if basin is filled)

To minimize such effects, we recommend filling the basin completely with clean water (not wastewater, not discharge water from other basins) and running the maximum permissible air throughput (see Section 1.9.2 / 1.9.3) through the aeration system for a period of 10 minutes daily. This is referred to as "maintenance mode" operation. If algae formation is observed, empty the basin immediately, clean the basin and its aeration system (see Section 1.10.2) and refill the basin with clean water. In the course of maintenance mode operation, lime, iron and other substances in the water may precipitate and deposit on the membranes. This depositing can impair the membranes' performance. We therefore recommend cleaning the membranes before taking them into continuous operation (see Section 1.10.1 / 1.10.2).

Operational Specifications

Properties of Air

The inlet air must be free of oil, dust, condensate and solvents. The intake air into the blowers must comply with the German Technical Guideline for Air Pollution Control (TA Luft) or equivalent industrial air purity regulations in your country. It must pass through suitable filters to remove suspended particulates. The temperature of the inlet air into the diffusers must be below the maximum permissible limits for their membrane materials (See Section 3, colour code: light green).

Air Throughput Through STANDARD and MAGNUM® Diffusers

Air throughput rates achieved through diffusers depend on the type and size of their membrane perforation. The table below gives maximum permissible air throughputs, expressed in the units [1 Nm³/h respectively SCFM per meter membrane length], for various perforation sizes.

Perforation /mm 0,6 1,0 1,2 1,4 2,0
Maximum throughput per meter of membrane length 6 Nm³/h/m 12 Nm³/h/m 16 Nm³/h/m 18 Nm³/h/m 24 Nm³/h/m
Maximum throughput per 39.3701 inch of membrane length 3,74

Air flow can be shut down completely if desired.

Air Throughput Through OTT D-REX® Diffusers

Air throughput rates achieved through diffusers depend on the type and size of their membrane perforation. The table below gives the maximum permissible air throughput, expressed in the units [m³/h respectively SCFM per disc], for 0.8 mm perforation.

Perforation /mm 0,8
Maximum throughput per disc diffuser 10 Nm³/h
6,23 SCFM

There is no minimum permissible throughput, i.e. air flow can be shut downcompletely if desired.

Use of Process Additives

Do not use any process additives (e.g. defoaming agents, acids, etc.) in your aeration system without having first obtained approval from OTT for the specific additive in question.

Design Considerations / Flow Conditions

Be certain to provide OTT all data and information relevant to water flow which can forcefully impact any diffusers in the basin. Sufficient clearance is required between the diffusers and nearby agitators. The required clearance is dependent on impacting flow speeds and the basin’s geometrical configuration as well as the drive power, design and location of the agitator. Clearance requirements must be established in discussions with OTT and the agitator manufacturer in the early stages of design of the aeration system.

Air Purge Intervals

Depending on the system, it may be necessary to air purge the diffusers from time to time. For this purpose, all diffusers are operated at maximum throughput for a short period of time. The maximum throughput for OTT tube diffusers can be found under 1.9.2, for OTT disc diffusers under 1.9.3.

Regular Maintenance and Inspection

Cleaning of Fouled Diffusers


The performance of diffusers is primarily dependent on the state of their membranes and their perforation. Wastewaters contain substances which can precipitate as fouling or deposits on the membranes. Examples include:

  • Lime scaling and carbonates
  • Iron and aluminium salts
  • Biofouling organisms
  • Polymers

The amount of fouling or deposits encountered on diffusers and membranes depends on the overall configuration of the wastewater treatment system, the operating conditions employed and the types and amounts of contaminants present in the wastewater.

Indications of Fouling

Fouling and incrustation are often indicated by the need for increased operating pressures in the diffuser system. Fouling is often accompanied by non-uniform, coarse bubble formation and reduced oxygen transfer efficiency.

Acid Dosing

Formic acid can be used to remove limescale fouling which obstructs air passages in the membrane. The required quantities and frequency of formic acid dosing must be determined in accor dance with the specific application at hand. Here too, the general rule of thumb is: »As much as necessary, yet as little as possible«.

We will be happy to advise you concerning our experience in this regard. Do not use formic acid or other acidic agents for diffuser cleaning without having first obtained approval from OTT for the specific substance or acidic agents in question.

Dosing of formic or acetic acid

The membrane of a compressed air aeration system may develop coverings and deposits depending on the origin and composition of the wastewater and the type and quantity of auxiliary substances used. An appropriate selection of the membrane material and contrasting maintenance and care measures can reduce these coverings.

Formic or acetic acid

If the covering is made of lime or carbonate, additional cleaning with the help of up to 60% concentrated technical formic acid (CH202) or up to 40% concentrated technical acetic acid (CH3C00H) is required, which is sprayed via the air volume flow.

Safety regulations

Please read all safety instructions and product datasheets regarding formic and acetic acid carefully beforehand. Ensure that all people who have access to the chemicals, including the chemical products suppliers, the security officer of the WWTPs, and the suppliers of all used devices (as dosing pumps, containers, etc.), are trained and informed on safety precautions.This document is not a complete instruction for dealing with formic and acetic acid. It explains the effect and use of formic and acetic acid to clean the membrane diffusers.

Basic workflow and effect

The formic or acetic acid is fed into the airflow of the aeration system to clean the membrane diffusers. Therefore,the airflow transports the obscured formic or acetic acid through the pipelines to the diffuser elements. The contact between the lime deposits and the formic or acetic acid arises at the perforation slots. The formic or acetic acid resolves the deposits and washes the perforation slots. It is not required to take the aeration system out of service and empty the aeration tanks.

Technical conditions

The aeration system needs to be intact and ready for operation. Blower, pipelines, and regulating elements are not allowed to exhibit defects. Formic or acetic acid may attack or destroy some materials.

Please consult with the suppliers of the components concerned before using formic or acetic acid and clarify whether they are resistant to formic or acetic acid. The more targeted the formic or acetic acid can be dosed, the greater the cleaning effect, and the more precise and economical the process. In addition, by selecting the appropriate dosing points, unnecessary contact of many components (e.g., butterfly valves) with formic or acetic acid is avoided.

Therefore, the dosing of formic and acetic acid should be actuated by suitable dosing pumps, which spray the acids in each soil pipe of the diffuser grid. For this purpose, each soil pipe must have a dosing connector. Dosing connectors are usually provided with a thread and are located behind the butterfly valves in the area of the arc of the soil pipes. The nozzle or the connection of the formic or acetic acid line is mounted on the dosing connectors. The formic and acetic acid must be stored and transported in suitable vessels. Mobile chemical containers with collection trays and wheels have proven their worth here. A dosing pump can be mounted on them. The connected hose line needs to be long enough to reach all connection points comfortably. If a regular or permanent dosage of formic or acetic acid is required, a permanently installed piping system can be installed as an alternative.

To check the effectiveness of the measure, the plant should have pressure gauges to measure the pressure in the piping system. Differential pressure and measuring device, which can precisely represent the cracking pressure of the membrane independently of the water column and without measuring the piping losses, is best suited.

Strength and amount of formic or acetic acid

Other acids or chemicals are forbidden because they may lead to substantial damages to the membranes or other components. This required strength of formic or acetic acid is chosen depending on the extent of the coverings and other individual technical conditions. The use of up to 60% concentrated technical formic or acetic acid or acetic acid of up to 40% is permitted. Lower concentrations are allowed. It is recommended to carry out tests, the success of which should be documented and evaluated. The objective has to be an optimized, plant-specific planning of the cleaning cycles following "as much as necessary, as little as possible".

The required amount of formic or acetic acid needs to be specified for each case and plant. Our guideline recommends obtaining at least 50 ml of formic or acetic acid per meter of installed membrane length. Whether it is necessary to dose the entire amount or if smaller doses achieve the desired effect can be observed individually.

Duration of a dosing interval

For more effectiveness of the acid, the duration of the dosing interval should not be chosen too short. As a guideline, we recommend using the amount of acid per meter aerator in ml as time in minutes.

10ml/m diffuser = 10 min Duration

Cleaning of Membranes in Empty Basin

General Information

When conducting walk-through inspections or working in basin, take care not to damage diffusers.

Spray Cleaning of Sludge Accumulation

Sludge accumulation on diffusers can be removed by water spraying. Do not use water spray with excessive impact force (i.e. from high-pressure water jets, fire-fighting hoses, etc.) as this will damage the membranes. Use a 3/4" hose to spray off accumulated activated sludge from the membranes. While cleaning diffusers by this method, air purge them as well at their maximum air throughput (given in Section 1.9.2). Following water spraying, air purge the diffusers at their maximum air throughput (given in Section 1.9.2) for a time period of 5 minutes.

Manual Cleaning of Membranes

Membranes may be wiped off or brushed off as long as this does not cause clogging of perforation or damage to the membrane. Do this using commercially available soft brushes used to clean automobiles. When cleaning diffusers by this method, air purge them at their maximum air throughput (given in Section 1.9.2) to blow out any foreign materials obstructing their perforation.

Permissible Cleaning Agents

Use only cleaning agents which have been approved in advance for use by OTT.

Dewatering of Piping

Use the air purge release valves of the headers to blow out condensate accumulation at regular intervals. The required time intervals are dependent on the piping system configuration, air throughput and other operating conditions as well as weather conditions. Check drainage traps of the main lines and drainage lines at regular intervals and empty as required.

Regular Checking of Bubble Pattern

Check the bubble pattern emitted from the diffusers at regular intervals using the air throughput typically used in operation. An increase in bubble diameter can be an indication of damage to membranes or fouling/clogging of the membranes.

Remediation Procedures in the Event of Membrane Damage

Should you discover irregularities in bubble patterns due to damage to the membrane, inform OTT immediately.

Maintain air throughput through the diffusers for as long as possible to minimize risk of wastewater or sludge intrusion into the aeration system.

Defective diffusers can be replaced underwater by industrial divers without disrupting operation. If repair is to be effected by raising the aeration system out of the basin, do not permit sludge or fouling (if any) on diffusers to dry out.

Replace defective diffusers in compliance with the applicable installation instructions, the instructions given above and other applicable specifications provided in this Product Handbook. In some situations, replacement of membranes only is the preferable method of repair. Always consult OTT for advice in this regard before beginning replacement.

If emptying the basin is unavoidable, maintain air throughput while draining the wastewater out of the basin, therefore avoiding sludge accumulation on the basin floor. Take care that any sludge accumulation or other fouling present on diffusers does not dry out. Avoid draining the basin if the ambient temperature is below freezing.


For the disposal of old or used diffusers, please contact a local disposal specialist, because the requirements usually vary between regions.

It is important to note that disposing of large amounts of biological or organic waste in the form of sludge and sediment should be avoided as much as possible. Therefore, the diffusers to be disposed of should be thoroughly cleaned beforehand.

From our experience, we can say that we usually have our containers declared as mixed construction waste with the disposal company.

Materials Used

  • Membranes: silicon/EPDM
  • Support pipes: polypropylene (PP)
  • Clamps + screw set: stainless steel
  • If applicable, packaging waste (cardboard, paper, film, wood)