Wastewater Engineering – Tertiary & Sludge Treatment (Week 14)

42 question-and-answer flashcards covering key points on tertiary wastewater treatment processes, nutrient control, and comprehensive sludge treatment, stabilization, and disposal methods.

What is the main objective of tertiary wastewater treatment?

To provide an advanced level of treatment that removes nutrients, toxic compounds, and additional organic matter/suspended solids before discharge to sensitive environments or for water reuse.

When is tertiary treatment typically required?

When the effluent will be discharged to a sensitive receiving water body or intended for reuse applications.

Name three broad categories of processes used in tertiary treatment.

Physical, chemical, and biological processes.

Which pollutants are primarily removed by chemical precipitation in tertiary treatment?

Phosphates, heavy metals, and some dissolved organics.

List three common chemicals used for chemical precipitation.

Alum, ferric chloride, and lime.

Why is phosphorus removal important in wastewater effluent?

To prevent eutrophication in receiving lakes and rivers.

In coagulation-settling-filtration, dual or multimedia filters are preferred over single-media sand filters because _.

single-media sand filters clog too easily.

What is the purpose of granular media filtration?

To remove suspended solids, turbidity, and sometimes microorganisms as a final polishing step before disinfection.

Give two typical media used in granular filtration.

Sand and anthracite (gravel may also be included).

Which membrane filtration type removes suspended solids and bacteria?

Microfiltration (MF).

Which membrane filtration type can remove viruses and macromolecules?

Ultrafiltration (UF).

Which membrane filtration type primarily removes divalent ions?

Nanofiltration (NF).

Which membrane process removes most salts and dissolved substances, producing high-quality effluent?

Reverse Osmosis (RO).

How does reverse osmosis work?

Pressure forces water through a semi-permeable membrane, leaving dissolved solids and contaminants behind (the process runs opposite to natural osmosis).

What percentage of total dissolved solids (TDS) and bacteria can reverse osmosis remove?

Approximately 95–99 % TDS and 99 % of bacteria.

Briefly describe electrodialysis.

A membrane process that uses an electrical potential to drive positive and negative ions through separate semipermeable membranes, leaving fresh water between them.

What is the main purpose of carbon adsorption in tertiary treatment?

To remove dissolved organic compounds responsible for color, odor, taste, and trace pollutants.

How is activated carbon "activated"?

By heating carbon to about 1500 °C, creating a very high surface area with extensive pore spaces for adsorption.

State two common nutrient-removal strategies highlighted for tertiary treatment.

Biological phosphorus removal and nitrogen control.

Define eutrophication.

An increase in nitrogen or phosphorus compounds that leads to excessive plant growth, oxygen depletion, and overall decline in water quality and aquatic life.

What share of a wastewater treatment plant’s construction cost is often attributed to sludge disposal facilities?

Approximately 40–60 % of construction cost (and about 50 % of operating cost).

Differentiate primary sludge from secondary sludge in terms of organic content.

Primary sludge contains about 70 % organic material, while secondary (biological) sludge is about 90 % organic.

Provide the formula relating sludge volume (V) to dry solids content (SC).

V₂ / V₁ = SC₁ / SC₂.

How much primary sludge (dry mass) is produced per capita per day for a wastewater concentration of 300 mg SS/L?

0.075 kg dry solids per capita per day.

For 200 mg/L BOD loading on secondary treatment, what is the typical mass of waste activated sludge produced per capita per day (dry basis)?

Approximately 0.03 kg/capita/day when 60 % of biomass is wasted.

Name the four major stages in a typical sludge management flowchart.

Preliminary treatment & digestion (stabilisation), conditioning & dewatering, utilisation, and disposal.

List the three main sludge thickening methods.

Gravity thickening, flotation thickening, and mechanical thickening.

Describe gravity thickening.

Dilute sludge settles and compacts in a circular tank; thickened sludge is withdrawn from the bottom while supernatant is returned to the plant.

What is Dissolved Air Flotation (DAF) thickening particularly useful for?

Separating small or light particles that settle slowly, and concentrating biosolids in plants <1000 PE.

Give two examples of mechanical thickening equipment.

Rotary Drum Thickener and Belt Thickener (centrifuges are also common).

What is the primary purpose of sludge stabilization?

To biochemically break down organic solids so they are less odorous, less putrescible, more dewaterable, and reduced in mass.

Name the two basic sludge stabilization processes.

Anaerobic digestion and aerobic digestion.

Aerobic digestion aims for what minimum percentage reduction in volatile solids?

About 38 % reduction.

What gas is principally produced during anaerobic digestion?

Methane (CH₄).

List the three general stages of anaerobic digestion.

Hydrolysis/fermentation, acidogenesis (acetogenesis & dehydrogenation), and methanogenesis.

What is sludge conditioning?

Treating sludge with chemicals (e.g., ferric chloride, lime, polymers) or heat so that water can be more readily separated during dewatering.

State the most common non-mechanised method of sludge dewatering.

Drying beds.

Give three mechanised dewatering options.

Belt press, centrifuge decanter, and filter press.

List three common sludge disposal methods.

Landfilling, ocean disposal, and incineration.

Mention two beneficial reuse options for treated biosolids.

Agricultural land application and composting (others include landscaping, reforestation, co-composting, etc.).

Why is land application of biosolids advantageous for soil?

It improves soil structure, moisture retention, aeration, and supplies macro- and micronutrients, partially replacing chemical fertilizers.

Before landfilling biosolids, what two sludge treatment steps are usually necessary?

Dewatering and often stabilization.

What is dedicated land disposal?

Applying heavy sludge loadings to a restricted land area set aside permanently with no crop production, often at reclaimed mining sites.