Tertiary Wastewater Treatment for Effluent Re-use

Welcome to a comprehensive guide on tertiary wastewater treatment for effluent re-use, a crucial step in ensuring water sustainability. While secondary treatment purifies water for discharge or agricultural use, it often isn't enough for unrestricted re-use. This article delves into the purpose and key processes of tertiary treatment, essential for communities facing limited water supplies.

Secondary treatment effluent, even at its best, still contains impurities such as finely suspended matter, dissolved chemicals (chlorides, nitrates, phosphates), and various bacteria, including pathogens. Tertiary treatment processes are designed to remove these remaining impurities, making the water safe for diverse applications, from industrial re-use to potential domestic consumption.

What is Tertiary Wastewater Treatment for Effluent Re-use?

Tertiary treatment serves as an advanced purification stage, acting as a "polishing" step after conventional primary and secondary treatments. Its primary purpose is to upgrade biologically treated sewage to a quality suitable for specific re-use applications or to meet stringent discharge standards. Depending on the intended re-use, the extent of impurity removal will vary, often mimicking processes used in natural water treatment for domestic consumption.

The typical processes involved in tertiary treatment include sand filtration, followed by disinfection (chlorination), and further maturation in ponds before the effluent is discharged or re-used.

Key Processes in Advanced Effluent Purification

To achieve high-quality effluent for re-use, several sophisticated processes are employed. These methods effectively target residual suspended solids, pathogens, and dissolved chemicals, significantly enhancing water safety and usability.

Sand Filtration for Enhanced Effluent QualitySand filtration is a physical and chemical process that plays a vital role in removing suspended and colloidal material, as well as bacterial contaminants, from the liquid. This process works through several mechanisms: adsorption, flocculation, sedimentation, and straining.

It provides a means of upgrading biologically treated sewage for re-use in industries and agricultural applications, and to achieve an effluent quality superior to gazetted standards. The removal of excessive suspended solids, which contribute to oxygen demand, naturally improves the overall quality of the effluent.

Various types of sand filters exist, including slow, rapid gravity, and pressure filters. The choice of filter depends on local conditions such as power availability, land area, manual labor, and the acceptable limit of suspended solids dictated by the effluent's end use.

It's crucial to understand that sand filtration is not a substitute for adequate biological treatment. For effective filtration, the effluent fed to filters should be settled, completely stable, and well nitrified.

Bacteriological Purification for Safe Water Re-use

Raw wastewater contains a high number of pathogenic and non-pathogenic organisms. While these numbers decrease through primary and secondary treatment, the final effluent often still exceeds legislative standards, necessitating additional treatment for bacterial removal.

Two main processes are generally used for bacteriological purification: chlorination and maturation ponds.#

Chlorination: Disinfecting Effluent

Chlorine is a potent oxidant that destroys pathogenic organisms when present in sufficient quantity, in the correct form, and for an adequate period. However, chlorine also reacts with organic substances and ammonia. Therefore, for effective disinfection, the effluent must be adequately stabilized beforehand to ensure the chlorine dose attacks microorganisms rather than being consumed by other reactions.

The objective is to introduce chlorine correctly, ensure proper mixing with the effluent, and provide a sufficient contact period in a tank. This allows the chlorine to react with organisms, minimizing free chlorine levels in the effluent leaving the tank. Breakpoint chlorination is particularly essential for the destruction of viruses, requiring full oxidation of all ammonia and complete satisfaction of the effluent's chlorine demand.#

Maturation Ponds: Nature's Polishing StepMaturation ponds are large earth ponds where final effluent undergoes further purification or "maturation." They serve as a polishing step to improve effluent quality before discharge or re-use, specifically targeting the removal of pathogens, coliforms, nutrients, and algae.

In maturation ponds, pathogenic bacteria and viruses are primarily inactivated through a complex reaction involving pH, temperature, and ultraviolet radiation from sunlight.

The importance of maturation ponds includes: * Continuing the natural purification process, improving effluent quality through a series of ponds (minimum 10-12 days total retention). * Acting as a buffer against malfunctions in the main treatment works. * Offering a high safety barrier against bacteriological pollution of receiving waters.

Maturation ponds should always be used in conjunction with well-purified humus tanks (secondary treatment) or sand-filtered effluent (tertiary treatment). They are not intended to compensate for under-designed or overloaded treatment plants, nor to cut operational costs.

Physical Features of Maturation Ponds

For optimal performance, maturation ponds should be divided into multiple smaller ponds, preferably 6 or 8, in series. This design eliminates short-circuiting and ensures maximum retention for purification.

In areas prone to bulrush growths, ponds may need to be 2m or 3m deep, with banks sloping down to full depth over a minimum distance compatible with stability.

Particular care is needed during construction. Walls between ponds should ideally exceed 3.5m in width for proper compaction and maintenance access, with bank slopes no steeper than 2 to 1. Walls, approximately 300mm above and below the water line, should be protected by rock-fill or concrete against wind erosion and water infestation. This lining also prevents mosquito breeding, avoiding costly weed control programs.

Ponds proper should not be used as irrigation dams, as their level must not fluctuate below design depths. A separate irrigation dam should be constructed, receiving only overflow from the last maturation pond. If irrigation is the sole disposal method, this dam needs a capacity equal to 14 days' average flow, maintaining a 12-day reserve capacity for wet weather storage.

Frequently Asked Questions about Tertiary Wastewater Treatment

Why is tertiary treatment necessary if secondary treatment is already applied?

Even after secondary treatment, effluent still contains fine suspended solids, dissolved chemicals like nitrates and phosphates, and a higher number of bacteria and viruses than typically allowed for safe re-use or discharge. Tertiary treatment provides the additional purification needed to meet stricter quality standards for re-use or environmental protection.

What are the main methods used in tertiary treatment?

The primary methods discussed are sand filtration for removing suspended and colloidal matter, and bacteriological purification, which includes chlorination for disinfection and maturation ponds for further pathogen inactivation and overall effluent polishing.

How do maturation ponds contribute to effluent re-use quality?

Maturation ponds enhance effluent quality by continuing natural purification processes. They inactivate pathogens through exposure to UV radiation, pH, and temperature, and help remove residual nutrients and algae. They also act as a buffer against treatment plant malfunctions, providing a crucial safety barrier.

Can sand filtration replace biological treatment?

No, sand filtration should not be considered a substitute for adequate biological (secondary) treatment. It is most effective when applied to effluent that is already settled, stable, and well nitrified from biological processes, as biologically poor effluent contains very fine colloidal material that is difficult to filter.

What is breakpoint chlorination and why is it important?

Breakpoint chlorination refers to adding enough chlorine to an effluent to oxidize all ammonia and satisfy the effluent's total chlorine demand. This process is crucial because it ensures sufficient free chlorine remains to effectively destroy viruses and other pathogenic organisms, leading to a higher level of disinfection.