Sewage & Sullage Treatment

Sewage and Sullage Treatment, Quantity and Characteristics of Waste Water

Sullage

Sullage denotes wastewater produced from domestic activities excluding toilet wastes - for example, wastewater from bathrooms, kitchens, washing places and wash basins. Sullage generally contains lower concentrations of organic matter and is less polluted than full sewage.

Sewage

Sewage refers to the liquid waste arising from domestic uses of water. It includes sullage, discharges from toilets and urinals, wastewater from commercial establishments, institutions and certain industrial effluents, together with any groundwater or stormwater entering the sewer system. Sewage undergoes decomposition that produces malodorous gases and contains numerous pathogenic organisms; it also typically has elevated concentrations of organic matter and suspended solids.

• Domestic sewage: Liquid waste from toilets, urinals, bathrooms, kitchen sinks, wash basins and similar domestic sources. This sewage is often highly foul due to human excreta.
• Industrial sewage: Liquid wastes from industrial processes such as dyeing, paper manufacturing, brewing and others. Industrial wastes may require separate treatment or pretreatment prior to discharge to municipal sewers.

Decomposition of Organic Matter in Sewage

Aerobic decomposition

When dissolved oxygen is available in sewage, biodegradable organic matter is oxidised by aerobic and facultative bacteria. These organisms use oxygen as the electron acceptor and convert organic matter to more stable, generally non-odorous end products.

Chemical conversions (representative):

Nitrogenous organic matter on oxidation:

NO₃^- + NH₃ + Energy

Carbonaceous organic matter on oxidation:

CO₂ + H₂O + Energy

Sulphurous organic matter on oxidation:

SO₄^2- + Energy

Anaerobic decomposition (putrefaction)

When dissolved oxygen is absent, anaerobic decomposition occurs. Anaerobic and facultative bacteria operating anaerobically convert complex organic matter into simpler reduced compounds; this process often produces odorous gases.

Chemical conversions (representative):

Nitrogenous organic matter on reduction:

N₂ + NH₃ + organic acids + Energy

Carbonaceous organic matter on reduction:

CH₄₊CO₂ + Energy

Sulphurous organic matter on reduction:

H₂S + Energy

Types of bacteria involved

• Aerobic bacteria: Thrive where dissolved oxygen is present and oxidise organic matter to stable end products.
• Anaerobic bacteria: Flourish in the absence of free oxygen and produce reduced, often odorous products.
• Facultative bacteria: Can operate either aerobically or anaerobically depending on oxygen availability.

Characteristics of Sewage

1. Physical characteristics

• Turbidity: Sewage is normally turbid and resembles dirty dishwater. Turbidity is measured using a turbidity rod or turbidity meter.
• Colour: A yellowish, grey or light brown colour indicates relatively fresh sewage; black or dark brown indicates stale sewage.
• Odour: Expressed as the Threshold Odour Number (TON), which represents the extent of dilution required to render the sample odourless to a trained panel.
• Temperature: Affects biological activity of microorganisms and the solubility of gases. Sewage temperature is often higher than ambient water temperature due to heat added during water use.

2. Chemical characteristics

• Total solids, suspended solids, settleable solids, dissolved solids:

  Total solids: Determined by evaporating a known volume of sewage and weighing the dry residue; expressed as mg/L.

  Suspended solids (SS): Solids retained by a filter of nominal 1 μm pore size.

  Dissolved solids (DS): Equal to total solids minus suspended solids.

  Total suspended solids: Consist of volatile solids and fixed solids. Volatile solids are the weight loss on ignition; fixed solids are the residue after ignition.

• Settleable solids: Determined using an Imhoff cone. The cone holds 1 litre and is graduated to about 50 mL. Sewage is allowed to settle for 2 hours and the settled volume is read directly in mL/L.
• pH: Indicates the acidity or alkalinity of sewage. Many treatment processes are pH-sensitive, so pH must be monitored and controlled if necessary.
• Chloride content: Typical chloride concentration in domestic sewage is about 120 mg/L. Higher values suggest mixing with industrial wastes. Chloride is commonly determined by titration with standard silver nitrate using potassium chromate as an indicator (Mohr method).
• Nitrogen content: Nitrogen in sewage indicates organic pollution and may occur as free ammonia, organic (albuminoid) nitrogen, nitrite and nitrate. The sum of free ammonia and organic nitrogen is referred to as Kjeldahl nitrogen. Free ammonia indicates nitrogen present before decomposition; nitrites indicate partial decomposition; nitrates indicate fully oxidised nitrogen. In drinking water, nitrates should be below 45 mg/L to avoid risks such as methaemoglobinaemia in infants.
• Fats, oils and greases (FOG): These interfere with treatment processes such as filtration and must often be removed by skimming or specialised pretreatment.
• Sulphides, sulphates and H₂S gas: Cause odour problems and may corrode concrete and metal sewer infrastructure.
• Dissolved oxygen (DO): Measured commonly by Winkler's method. Receiving waters should have a DO above about 4 mg/L to protect aquatic life; sewage discharged with low DO can reduce dissolved oxygen in the receiving stream. DO is inversely related to temperature (higher temperature → lower DO solubility). DO solubility decreases with increase in temperature and salinity.
• Chemical oxygen demand (COD): The amount of oxygen required to chemically oxidise organic matter to CO₂, H₂O and other oxidised species; COD is measured by strong chemical oxidants rather than biological respiration.
• Total organic carbon (TOC): Another way to express organic content by measuring carbon concentration. TOC can be calculated from known concentrations of individual organic components.

Biochemical Oxygen Demand (BOD)

BOD is the amount of oxygen required by microorganisms to decompose the biodegradable organic matter present in wastewater under specified conditions.

BOD₅ (definition):

The standardised BOD test commonly used is the five-day BOD at 20 °C, denoted BOD₅. Empirically, BOD₅ is taken as approximately 68% of the ultimate BOD for many wastewaters under normal conditions.

This implies: BOD5 ≈ 0.68 L  ⇒  L ≈ 1.47 BOD5

BOD₅ = D.O. consumed by diluted sample ×

= D.O. consumed × dilution factor

L_t = L at t = 0

L_t =  Amount of organic matter Present at time t

L = Ultimate BOD(BOD_u)

∝L_t

Integrating both sides, we get

Log_e L_t = - Kt + C

At t = 0, L_t = L C = log L

log L_t = log L - K_t

Loge  = - Kt

where K_d = K/2.3

BOD at time t = L - L_t = Y_t = L - L* 10^{- K}_dt

Y_t = L [1 - ₁₀- K_d t ]       K_D = De-oxygenation constant

The value of K_d changes with temperature and this relationship is approximately given by the equation

The value of K_d changes with temperature; a commonly used relationship is of the form K_dT = K_d20 × θ^(T-20), where θ is a temperature coefficient (typical θ values ≈ 1.03-1.06 depending on conditions).

Population Equivalent

Population equivalent (PE) is a way to compare industrial wastewater strength to a standard per-capita domestic sewage load. It is used for charging industries and for estimating load on municipal sewage treatment plants. The PE of an industrial effluent is the number of inhabitants that would produce the same pollution load (commonly expressed in terms of BOD).

Relative Stability

Relative stability (S) is defined as the ratio (expressed as a percentage) of the oxygen already present in the effluent to the total oxygen required to satisfy its first-stage BOD demand.

Empirical expressions for relative stability are given by:

S = 100 [1 - (0.794)^t₂₀]

or

S = 100 [1 - (0.630)^t₃₇]

where t₂₀ and t₃₇ are the times (in days) for a sewage sample to decolourise a standard volume of methylene blue solution when incubated at 20 °C or 37 °C respectively.

Notes on Measurement and Practical Points

• Many parameters have standard test methods (APHA/IS/ISO). For example, DO by Winkler method, BOD by dilution and incubation, COD by chemical oxidation and reflux, suspended solids by filtration and weighing.
• Pretreatment of industrial effluents (e.g., removal of FOG, neutralisation, removal of toxic substances) is often required before discharge to municipal sewers to protect biological treatment processes.
• Control of pH, temperature and aeration are key operational parameters for biological treatment units (activated sludge, trickling filters, ponds, etc.).
• Design and operation decisions (e.g., detention times, sludge handling, aeration requirements) are based on the characteristic wastewater concentrations such as BOD, COD, SS, nitrogen and phosphorus levels.