Planning Foundations for a Wastewater Treatment Plant

Wastewater treatment plants require adequate ventilation technology to supply their aeration tanks, and this presents exacting technical specifications and design parameters to match the dynamic air supply requirements, especially because it is important to maintain energy efficiency and reduce overall plant running costs. Above all, the installed equipment must be of the highest standards of reliability and freely available. As all engineers know, a chain is only as strong as its weakest link. This article provides basic data on how to design machines, rooms and pipeline routes for such an application.

Calculation of Working Data

Standard state (physical) of a gas is defined as the pressure pNabs = 1,01325 bar, when the temperature is TN = 273 K - 0 °C. For instance, some parameters taken in standard state include:

• the volume flow in standard state Q_N [m_N³/h]
• the density in standard state: ρN [kg/m³]
• the relationship between the temperature in degrees Celsius t[°C] T=TN+ t [K] and the thermodynamic temperature T[K]

Suction State

The blowers/compressors are built for use in the intake state. This refers to the real-time pressure in the socket for the inlet represented as p_1abs[bar], at either average or maximum temperature for the gas used at the inlet, namely, t₁[°C] or T1=TN+T₁[K].

In order for atmospheric air to be sucked in, the machine must exert an average absolute intake pressure p_1abs = 1.0 bar, taking the intake temperature t₁ = 20 °C or T₁ = 293K.

Impact of the Installation Height

Image Credit: Aerzen

Calculation of Volume Flow Q From Prescribed Mass Flow ṁ

This is represented in the following equations:

At standard state,

Where the mass flow is denoted by m ̇ ̇[kg/h] and the density ρ in [kg/m³]

And at intake state,

Density ρ_n of gases in standard state and specific heat capacity c_p is shown as follows:

Calculation of intake volume flow Q₁ from prescribed standard volume flow Q_N

1.	Dry gas
2.	Humid gas

Equation 1 is used to convert for the transport and compression of atmospheric air, which has a relative humidity of about 60% on average and whose installation height may be up to 500 mNN, with adequate precision.

Calculation of density “p₁“ in intake state

1.	Dry gas
2.	Humid gas

Density “p₁“ in intake state of atmospheric air p_1abs = 1.0 bar

Partial pressure of steam, saturation state

The key to the above terms is given in the following box for convenient reference:

Image Credit: Aerzen

Explanations of Individual Standards for Performance Measurement and Calculation of the Standard Volume Flow

Image Credit: Aerzen

The Power Requirement and Its Characteristics

Different levels of performance may be described, which vary with the expected losses at each specification. Of course, as the complexity of a system, packaged unit or plant increases, so does that of its performance specifications and it also becomes increasingly challenging to conduct a performance comparison.

The following comparison chart relates the performance to the model type for various blowers/compressors. The power required for each type increases from the units which show mechanical shaft performance at the stage to the completely powerized unit.

Image Credit: Aerzen

Explanations for ISO 1217

Aerzener Maschinenfabrik GmbH has published method specified for positive displacement blowers at each level of performance, measured in terms of the volume flow and power requirements. This includes parameters for operating and test conditions which must be complied with to ensure a full performance evaluation.

Compressors which have a single fixed speed may be produced as a batch or in series, and are engineered to meet specified levels of performance. These are subjected to a test run as described in annexures B, C and D, based upon which type and design the compressor conforms to.

Variable speed compressors are also produced the same way with specific values of operational performance, and their test runs must meet the standards in annexure E.

Overall, volume flow may be defined as contained in 3.4.1, which specifies it as “volume flow measured at the discharge nozzle and calculated back to the conditions of the suction side”. In annex C, packages units which have a fixed speed to produce compressed air or nitrogen are described, with the maximum permissible deviations being set forth in chart C2 below:

The a./m. chart shows tolerance values applicable for all manufacturing and measurement tolerances. Annex 2.4 specifies that electrical compressors must be fully mounted packaged units under customer-specified conditions at the time of measurement, and they should be assessed by the terminal power they produce.

Annex E describes a compressor with frequency converter which must follow the same regulations and has the same tolerances as fixed-speed compressors.

Explanation of PTC 10 – 1997

The PTC 10 – 1997 is a standard for units used in North and South America, and sets forth the operating procedure to quantify the thermodynamic power for both axial and centrifugal compressors as well as blowers or fans, once specified conditions are met.

ASME PTC 10 Acceptable Deviations for Test Parameters with Comparable Conditions

Explanations of ISO 5389

The ISO 5389 deals with test conditions for test runs of centrifugal compressors which are electrically driven, with a drive power of 75-1865 kW.

The maximum permissible deviations are defined as follows:

The ISO 1217 standard has, to a large extent, been shaped by AERZEN with over 150 years of displacement blower manufacturing history, while the PTC 13 is still being refined. AERZEN being a German company performs tests to this standard for its displacement blowers.

Most wastewater treatment plants are fitted with frequency converters today, and such upgrades can be guided by AERZEN with its range of low-cost devices. Here again the company must keep up with the requirements for efficiency and performance, as well as the need to compare favorably with other brands on the market.

For instance, if a wastewater treatment plant has a power rating that has ISO 1217 tolerance standards, it must meet annex E specifications for both a new compressor and a frequency converter. Here it is important to remember that high-end frequency converters are already part of the plant equipment in most cases, and therefore the customer should be given this freedom to choose only such machines as are necessary. For this reason, AERZEN provides customized solutions to fit each project individually. The range of such flexibility includes:

• Separate blowers and compressor stages
• Packaged units with/without motors
• Packaged units with/without external frequency converters so that they can be installed separately or with the rest of the equipment
• Packaged units with internal frequency converter

The products supplied are the basis upon which the standard or tolerance specification is applied. For example, the ISO 5389 is the European standard for turbo compressor packages but with different tolerance ranges. Though a German company, AERZEN must keep different standards and tolerances in mind because it supplies its products globally and has a wide range of products displaying many different types of compressor technologies designed particularly for wastewater treatment plants. The company uses the standard tolerance for all its products so that it can guarantee the highest energy optimization and safety standards whether the project uses only AERZEN machines or combines them with other brands – a learning amassed from 150 years of experience in supplying compressors from the very beginning of the growth of the wastewater treatment industry.

Applied Standards in Wastewater Technology Regarding Standard Volume Flow

Wastewater treatment technology uses several applied standards to calculate the standard volume flow, the most important being:

1. DIN ISO 1343: volume flow in standard state where T1 =273 K, p1 =1,013 bar, rF=0%

2. ISO 2533: volume flow in standard state where T1 =288 K, p1 =1,013 bar, rF=0%

3. ISO 1217: volume flow in standard state where T1 =293 K, p1 =1,000 bar, rF=0%

For example, see the following table:

t₁= Inlet temperature

Once the initial consultation with AERZEN engineers is completed they will be able to design the best package in accordance with the best power rating that conforms to the client’s standard or tolerance of choice.

This information has been sourced, reviewed and adapted from materials provided by Aerzener Maschinenfabrik GmbH.

For more information on this source, please visit Aerzener Maschinenfabrik GmbH.