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IS 7090:1985 is the Indian Standard (BIS) for guidelines for rapid mixing devices. This standard provides guidelines for the design and selection of rapid mixing devices used in the coagulation stage of water treatment plants. It details the purpose of rapid mixing, outlines various types of devices (mechanical, hydraulic), and establishes key design criteria like velocity gradient (G), detention time (t), and the Gt value.
BIM-relevant code. See the BIM Hub for ISO 19650, IFC, and LOD/LOIN frameworks used alongside it.
Practical Notes
! The primary goal is the near-instantaneous dispersion of coagulant; achieving a precise Gt value is secondary to preventing short-circuiting and ensuring uniform chemical distribution.
! For mechanical mixers, the impeller type (e.g., radial flow turbine) and its position within the tank are critical to creating the correct flow patterns and avoiding dead zones.
! Hydraulic methods like baffled channels or hydraulic jumps are effective but offer less operational flexibility compared to mechanical mixers, as mixing energy is dependent on flow rate.
Ten States Standards 2022 EditionGreat Lakes-Upper Mississippi River Board (GLUMRB), USA/Canada
HighCurrent
Recommended Standards for Water Works
Provides specific design criteria for rapid mix units within the broader context of water treatment plant design.
AWWA Manual M37, 3rd Ed.American Water Works Association (AWWA), USA
HighCurrent
Operational Control of Coagulation and Filtration Processes
Offers detailed operational and design guidance for the entire coagulation process, including rapid mixing.
Key Differences
≠IS 7090 is more prescriptive, specifying ranges for impeller speeds and tank dimensions, while international guides focus on achieving performance targets (G value, detention time) regardless of specific equipment geometry.
≠Modern international standards (e.g., Ten States) recommend higher minimum velocity gradients (G > 700 s⁻¹) compared to the IS code's minimum of 300 s⁻¹, reflecting a trend towards more intense mixing.
≠The acceptable detention time in IS 7090 (20-60 seconds) is longer than current international best practice, where high-energy mixers achieve optimal results in under 10 seconds, and sometimes as low as 1-3 seconds.
≠IS 7090:1985 focuses primarily on mechanical back-mix reactors. Modern equivalents like AWWA M37 provide extensive coverage of alternative technologies like in-line static mixers and motor-driven in-line blenders.
Key Similarities
≈All standards are based on the fundamental principle that rapid, high-energy, and uniform dispersion of coagulant is essential for effective water treatment.
≈The concept of the velocity gradient (G), developed by Camp and Stein, is used universally as the primary design parameter for quantifying mixing intensity.
≈Both the IS code and international guidelines mandate the use of baffles in mechanically agitated tanks to prevent vortex formation and ensure complete, homogenous mixing.
≈The need for system redundancy, such as providing standby mixing units or chemical feed pumps, is a shared principle to ensure reliability and continuous operation of the treatment plant.
Parameter Comparison
Parameter
IS Value
International
Source
Velocity Gradient (G)
Not less than 300 s⁻¹
Typically 700 - 1000 s⁻¹
Ten States Standards 2022
Detention Time (t)
20 to 60 seconds (optimally < 30 s)
Less than 30 seconds; often < 10 seconds for high-energy systems
AWWA Manual M37
Overall Mixing Metric (Gt)
Implied range: 6,000 - 18,000
Often targeted in the range of 10,000 to 20,000, but with a strong preference for high G and low t
AWWA Manual M37
Tank Geometry
Recommends square tanks with depth 1.0 to 1.3 times width.
Square, circular, or in-line configurations are acceptable based on performance.
Ten States Standards 2022
Baffle Width
1/10 to 1/12 of tank side width
Approximately 8-10% of tank diameter or side width (approx. 1/12)
Ten States Standards 2022
Mixer Shaft Speed
Prescribed ranges (e.g., Turbine: 100-150 rpm)
Not prescribed; calculated based on desired G, impeller choice, and tank volume.
AWWA Manual M37
⚠ Verify details from original standards before use
Key Values5
Quick Reference Values
Recommended Velocity Gradient (G)700 to 1000 s⁻¹
Recommended Detention Time (t)20 to 60 seconds
Recommended Gt Value (Camp Number)20,000 to 60,000
Minimum Velocity Gradient (G)300 s⁻¹
Typical Baffle Spacing in Baffled ChannelNot less than 450 mm
Key Formulas
G = sqrt(P / (μ * V)) — Velocity Gradient (Camp and Stein equation)
Tables & Referenced Sections
Key Tables
Table 1 - Recommended Design Parameters for Rapid Mixers
To achieve instantaneous and uniform dispersion of coagulant chemicals throughout the raw water to destabilize colloidal particles before flocculation (Clause 3).
What is a typical detention time for a rapid mix unit?+
Generally between 20 to 60 seconds. Modern practice often favors shorter times (under 30 seconds) for efficient chemical dispersion (Clause 5.3).
What is the recommended range for the velocity gradient (G)?+
Typically between 700 and 1000 per second (s⁻¹), although values as low as 300 s⁻¹ can be used depending on the water quality and coagulant (Clause 5.2).
What are the common types of rapid mixers?+
The code describes mechanical mixers (back-mix and plug-flow types), baffled channels, hydraulic jumps, and pump/diffuser systems (Clause 4).