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What is the Composition of Wastewater?

By: Haseeb Jamal / On: Apr 27, 2017 / Definition, Shallow

Constituents of Waste Water

Constituents of Waste Water are characterized in terms of its physical, chemical and biological composition

Physical Characteristics

Solids content

  • Floating matter
  • Settleable matter
  • Colloidal matter
  • Matter in solution

Particle size distribution; Turbidity; Color; Transmittance; Temperature; Conductivity; Density; Specific gravity; Specific Weight

Solids classification

Solids interrelationships

Settleable solids: Placing 1‐L sample in Imhoff cone and noting volume of solids in mm that settle after 1 h; Typically 60% of suspended solids (SS) in municipal wastewater are settleable

Total solids (TS): Obtained by evaporating wastewater sample to dryness (at 103‐ 105°C) and measuring mass of residue

Total suspended solids (TSS): Filtration step is used to separate TSS from total dissolved solids (TDS); Portion of TS retained on filter (e.g., Whatman fiber glass filter‐GF/C) measured after being dried at 105°C

Total Suspended Solids (TSS)
 

More TSS measured as pore size of filter used is reduced;

Important to note filter paper pore size, when comparing TSS values;

TSS and BOD universal effluent standards by which performance of treatment plants is judged for regulatory control purposes

Total Dissolved Solids (TDS)

Solids contained in filtrate that passes through a filter with nominal pore size of 2 μm or less are classified as dissolved; Size of colloidal particles in wastewater typically in range from 0.01‐1 μm

Volatile and Fixed Solids (VS and FS)  Material volatilized and burned off when ignited
at 500 ± 50oC classified as volatile solids (VS);

In general, VS are organic matter

Residue that remains after sample is ignited at 500 ± 50oC classified as fixed solids (FS);

TS, TSS, and TDS comprised of both VS and FS  Ratio of VS to FS used to characterize wastewater with respect to amount of organic matter present

Particle Size Distribution (PSD)

To understand nature of particles that comprise TSS in wastewater, measurement of particle size is undertaken

PSD important in assessing effectiveness of treatment processes (secondary sedimentation, effluent filtration, and effluent disinfection)

PSD methods can be divided into two general categories:

  1. Methods based on observation and measurement
  2. Methods based on separation and analysis techniques

Commonly used methods for particle size analysis:

1. Serial filtration: Wastewater sample is passed sequentially through series of membrane filters with circular openings of known diameter, and amount of suspended solids retained in each filter is measured.

Electronic Particle Counting

  • Particles in wastewater are counted by diluting a sample and then passing diluted sample through calibrated orifice or past laser beams;
  • As particles pass through orifice, conductivity of fluid changes, owing to presence of particle. Change in conductivity is correlated to size of equivalent sphere;
  • Similarly, as particle passes by laser beam, it reduces intensity of laser because of light scattering. Reduced intensity is correlated to diameter of particle. Particles counted are grouped into particle size ranges. In turn, volume fraction corresponding to each particle size range is computed.

Microscopic Observation:

Placing small wastewater sample in particle counting chamber and counting individual particles;

  • To aid in differentiating different types of particles, various types of stains are used;
  • In general, microscopic particle counting is impractical on routine basis;
  • However, it can be used to qualitatively assess nature and size of particles in wastewater

The typical composition of wastewater based on strength. The important characteristics measured in wastewater included...

  • Biochemical Oxygen Demand (BOD) [100-300 mg/L as O2]
  • Suspended solids (SS) [100 – 350 mg/L]
  • Settleable solids [5-20 mL/L]
  • Total Kjeldahl nitrogen (TKN) [20-80 mg/L]
  • Total Phosphorus [5-20 mg/L as P]

A typical solids analysis of wastewater, of the total solids, 50% is dissolved, 50% suspended. Of the suspended solids, 50% will settle. Industrial activity changes the composition of wastewater, often introducing toxic substances such as chromium and cadmium from plating operations.

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