Introduction

In this second installment of our Sewage Treatment Parameters Education Series, we’ll focus on Suspended Solids (SS). This parameter is closely linked to Biochemical Oxygen Demand (BOD) and is a key indicator of effluent quality.

What are Suspended Solids?

Suspended Solids (SS) refer to the visible particles in sewage that do not easily settle out over time.

• Definition:
  A measurement of the solids suspended in sewage (influent or effluent), typically expressed in mg/l or ppm (parts per million).

Why Are Suspended Solids Important?

1. Effluent Quality:

   • Effluent with low SS levels appears clear, while higher concentrations make the water increasingly cloudy.

   • The European discharge standard for SS is 35 mg/l, forming part of the standard '25/35' effluent quality:

     • BOD ≤ 25 mg/l

     • SS ≤ 35 mg/l

2. Impact on BOD:

   • A significant portion of BOD is attached to suspended solids.

   • Reducing SS also contributes to a corresponding reduction in BOD.

Typical SS Levels

• Influent SS (Untreated Sewage): ~300 mg/l.

• Effluent SS (Treated Sewage): Must be reduced to ≤35 mg/l or lower, depending on regulations.

How Are Suspended Solids Reduced?

1. Physical Methods:

   • Settlement: Sedimentation tanks and other mechanical processes separate solids from water.

2. Biological Treatment:

   • Biological processes in sewage treatment plants break down and further remove solids.

3. Advanced Filtration (for stricter standards):

   • Saran Filters or Final Effluent Polishing Filters (FEPF) can reduce SS concentrations to very low levels, ensuring crystal-clear effluent.

Suspended Solids are a critical parameter in sewage treatment, closely tied to BOD reduction and effluent clarity. Achieving the '25/35' standard is a core objective for most package sewage treatment plants.

Next Topic: Population Equivalent (PE)

Stay tuned for our next post, where we’ll discuss Population Equivalent (PE) and its role in designing sewage treatment systems.

Questions?

If you have specific questions or topics you’d like us to explore, feel free to contact us. We’re always happy to help!

About BMS

BMS is a quality manufacturer of package products for wastewater and stormwater treatment:

• Established: 1986

• Global Reach: Exported to over 50 countries.

• Family Business: A third-generation company with deep expertise.

BMS Product Range

BMS provides a wide array of surface water and wastewater treatment products, including:

• Attenuation systems

• Interceptors

• Holding tanks (chemical-resistant options available)

• Hydrobrakes

• Rainwater harvesting tanks

• Vortex silt/oil/debris separators

• Fully fitted pumping stations

• Recyclone: A plug & play vehicle wash water recycling system

View the full product catalogue here

Additional Services by BMS

• Free Design Service: For sewage treatment, pump stations, and surface water management.

• Operation and Maintenance: Full services for existing installations, including repair, advice, and testing.

• Free CPD Presentations: Available for sewage treatment and surface water management, delivered at your location.

We would be delighted to discuss any requirements you might have, and we are available to meet you at your convenience.  Contact us today.

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