Sludge accumulation in polishing ponds treating anaerobically digested wastewater

Area of Science:

• Environmental Engineering
• Wastewater Treatment Technologies
• Solid Waste Management

Background:

• Wastewater treatment ponds accumulate bottom sludge from settleable solids, reducing effective volume and necessitating periodic removal.
• This sludge can originate from influent wastewater or form via algal flocculation during treatment.
• Polishing ponds are often used for post-treatment of effluent from Upflow Anaerobic Sludge Blanket (UASB) reactors.

Purpose of the Study:

• To quantify bottom sludge accumulation in a polishing pond treating UASB effluent.
• To assess the characteristics and composition of the accumulated bottom sludge.
• To determine the necessity of desludging operations for polishing ponds.

Main Methods:

• A pilot-scale polishing pond was operated for one year, treating approximately 60 m³ of UASB effluent per m³ of pond volume.
• Bottom sludge mass and composition were evaluated after the operational period.
• Analysis included volatile solids, nitrogen, phosphorus, helminth eggs, and faecal coliforms.

Main Results:

• A total of 70 g of bottom sludge accumulated per m³ of treated wastewater.
• Approximately 50% of the sludge originated from influent solids, with the other 50% from in-pond algal settling.
• The bottom sludge exhibited high volatile solids (58%), macronutrient content (3.9% N, 1.1% P), but poor hygienic quality (high helminth eggs and faecal coliforms).

Conclusions:

• Bottom sludge growth in polishing ponds is sufficiently low to likely avoid desludging during the pond's lifespan.
• Utilizing a polishing pond for post-treatment can eliminate the need for excess sludge discharge from UASB reactors, reducing operational costs.
• While stable and suitable for drying, the bottom sludge poses significant hygienic risks requiring careful management.