Water Treatment

3.2 Wastewater Treatment Processes

3.2.1 Preliminary Treatment

The primary goal of preliminary treatment is to eliminate large solids and coarse materials from raw wastewater, which is critical for the efficient operation and maintenance of subsequent treatment processes. This stage includes operational methods such as coarse screening, grit removal, and the comminution of oversized objects. In grit chambers, proper water velocity or the use of air ensures that organic solids do not settle. However, smaller wastewater treatment plants typically do not include grit removal in this phase. Comminutors may be utilized to break down large particles, allowing for their removal as sludge in later processes. Notably, flow measurement devices, such as standing-wave flumes, are essential components integrated into the preliminary stage to monitor flow rates.

3.2.2 Primary Treatment

Primary treatment focuses on the sedimentation of settleable organic and inorganic solids and the removal of floating materials (scum) via skimming. During this phase, approximately 25% to 50% of the incoming Biochemical Oxygen Demand (BOD5), 50% to 70% of Total Suspended Solids (SS), and about 65% of oil and grease are removed. Additionally, organic nitrogen, phosphorus, and heavy metals are also partially removed, though dissolved and colloidal constituents remain largely unaffected. The effluent resultant from this treatment is termed primary effluent, typically processed in sedimentation tanks or clarifiers. These units can be round or rectangular, with depths ranging from 3 to 5 meters and hydraulic retention times of 2 to 3 hours. The settling solids, known as primary sludge, are collected from the tank's bottom by sludge rakes, while scum is removed from the surface.

3.2.3 Secondary Treatment

Secondary treatment serves as an extension of primary treatment, aiming to remove residual organics and suspended solids. This process predominantly utilizes aerobic biological treatment, where microorganisms (primarily bacteria) metabolize organic materials, generating additional microorganisms and releasing inorganic by-products such as CO2, NH3, and H2O. Various aerobic biological processes are employed, differing mainly in the oxygen supply method and organism metabolism rates. High-rate treatments involve smaller reactor volumes and higher microorganism concentrations, enhancing the growth rate of new organisms. Following treatment, microorganisms are separated from the treated wastewater via sedimentation in secondary clarifiers. The biological solids generated during this sequence, referred to as secondary or biological sludge, are typically combined with primary sludge for further processing. Established high-rate processes include activated sludge systems, trickling filters, oxidation ditches, and rotating biological contactors (RBC).

i. Activated Sludge

In the activated sludge method, wastewater and microorganisms are mixed in an aeration basin where vigorous aeration occurs, promoting oxygenation and slurry mixing. Hydraulic retention time ranges from 3 to 8 hours. Post-aeration, separation occurs via sedimentation, yielding clarified secondary effluent, where a portion of the biological sludge is recirculated and the excess undergoes processing. Variations like extended aeration and oxidation ditches are also prevalent.

ii. Trickling Filters

Trickling filters consist of media-filled basins where wastewater intermittently trickles over surfaces colonized by microorganisms that metabolize organics. Oxygen is supplied naturally through air flow or mechanical means, facilitating the treatment process. Sloughed material is separated in a secondary clarifier, contributing to the sludge processing stream.

iii. Rotating Biological Contactors

RBC systems feature rotating discs coated with microorganisms immersed in wastewater. Oxygenation occurs during the rotation, allowing the biofilm to treat the organic content efficiently. Sloughed biofilm is similarly processed as described for trickling filters.

3.2.4 Tertiary and/or Advanced Treatment

Tertiary treatment, or advanced treatment, is employed to address residual wastewater contaminants that remain post-secondary treatment. This stage often incorporates additional processes for nitrogen, phosphorus, suspended solids, heavy metals, and other specific needs, effectively polishing the effluent for discharge.

Wastewater Characteristics

Wastewater, which includes all used water requiring treatment before returning to natural water systems, originates from various sources, including domestic, industrial, agricultural, and stormwater runoff. The treatment process is vital to prevent water pollution and contamination.

Wastewater Treatment Techniques

Primary Treatment Characteristics

  • Screening: Utilizes screens of different sizes to remove large debris and larger particles like grit and gravel, employing horizontal, vertical, or wire mesh screens.

  • Skimming: This process removes oil and scum from the wastewater surface.

  • Sedimentation: Settles solids via mechanical and chemical processes, often employing sedimentation tanks to achieve effective separation.

  • Neutralization and Equalization: These steps are used to adjust pH levels and ensure consistent characteristics in wastewater before advanced treatment is deployed, maximizing treatment efficacy.

Secondary Treatment Objectives

Secondary treatment is fundamental for further reducing pollutants, utilizing aerobic biological processes. The growth of microorganisms in controlled environments promotes the breakdown of organics, culminating in effluent subjected to secondary clarifiers to achieve the removal of biological and organic solids.

The Role of Microorganisms

Bacteria and algae cooperate within oxidation ponds, which serve as an ecological method for treating wastewater. Understanding the interactions between these organisms is crucial for optimizing treatment processes, particularly their roles in oxygen production and waste decomposition.

Advanced Treatment Methods

Advanced treatments such as ion exchange and chlorination play critical roles in removing contaminants not adequately treated by earlier methods.

  • Ion Exchange: Involves replacing undesirable ions in wastewater with harmless ones via resin systems, which are adjusted based on solution characteristics.

  • Chlorination: A common final step to disinfect treated water, ensuring pathogen elimination before distribution.