Public markets present special challenges for wastewater system designers. There are frequently multiple sources of sewage, including those from the wet section (fish, poultry and meat preparation and vegetable washing), food preparation areas and public toilets. Additionally, markets can be quite large, sometimes occupying several hectares or more. This can complicate the planning and installation of the sewer collection system, or the pipes that transport the sewage from the source of generation to the point of treatment. Finally, public markets generally have very limited space, requiring treatment works to be located underneath parking areas, or remote areas of the market grounds. Understanding the associated costs, as well as opportunities for full cost recovery, is important in developing long-term and sustainable solutions for wastewater treatment for public markets.
The cost of developing wastewater treatment systems for public markets includes
- the sewer lines and wastewater collection system;
- pretreatment devices for the removal of trash and grease;
- treatment plants and equipment, including the tanks, pumps and controllers that are used for processing wastewater;
- the equipment that allows for reuse and recycling of the treated wastewater;
- long-term operation and maintenance requirements, including staffing and electricity.
Contents
1. Sewer lines and collection systems
Sewer lines that transport the wastewater from market stalls to the treatment plant may be a significant expense in the overall wastewater treatment project. Public markets often occupy large spaces that may require hundreds or thousands of meters of sewer pipes. Due to distance or where gravity alone is not sufficient, sometimes pumping systems are required to deliver the sewage to the treatment works.
To understand the specific requirements of the sewage collection system for a market, use the market site plan and identify the following:
- all locations where wastewater discharges from the market—the wet section, food preparation area, public toilets, and other areas location of the proposed wastewater treatment site
- topography or any slopes (gravity sewer lines require slopes of 2% to keep the sewage flowing, and long sewer pipe runs require deep excavations or lift stations areas that will utilize the treated effluent), toilet flushing, and street cleaning
2. Pretreatment devices
Grease and trash must be removed from the wastewater before it enters the sewer pipes. If not removed quickly, grease from the food preparation areas clogs sewer lines and treatment works. Trash, including vegetable scraps, fish scales, meat trimmings and similar material, must also be removed from the wastewater prior to treatment for similar reasons. Install properly sized grease and trash traps at key discharge points throughout the market area. Scheduled daily cleaning of these traps is the best way to keep sewer pipes clear.
3. Effluent reuse and recycling
There is increasing interest in using reclaimed wastewater effluent for beneficial reuse to defray costs and recycle this valuable resource. Some reuse options include toilet flushing, road washing and dust suppression. In order to reuse treated wastewater effluent, it must first be filtered and then disinfected. Filtration systems commonly use multimedia filled canisters that receive the wastewater under pressure. These high rate filters can remove virtually all of the suspended material in the effluent. Only after filtration can effluent be properly disinfected.
There are three categories of disinfection that are suitable for market wastewater flows:
- Chemical: Chlorine is a common chemical disinfectant for wastewater effluent. Erosion style chlorinators use the chlorine in solid form as chlorine blocks or tablets. Similar to swimming pool chlorinators, the wastewater effluent flows over the solid chlorine tablets and dissolves or erodes the chlorine, delivering the chemical to the effluent. This is an effective method for small flows. Also popular are liquid chlorinators where chlorine bleach (sodium hypochlorite) is added to water and pumped into the wastewater stream by a small metering pump. These are easy and inexpensive to operate.
- Radiation: Ultraviolet light is a strong disinfectant and can kill most bacteria found in wastewater. While initially more expensive than chlorinators, these methods save in the long run as they require much less operation.
- Physical: Ozone is a highly reactive gas that kills pathogens on contact. Ozone generators are simple devices that produce the gas and then bubble it through the effluent to react with and kill pathogens. Ozonated effluent is especially clear and ideal for use in toilet flushing as the process also helps eliminate odors typically associated with treated wastewater effluent.
Whichever method is used, carefully consider the initial installation costs, power consumption and power reliability, as well as replacement costs and operator requirements prior to selecting the equipment.
4. Full cost recovery
Recovering the costs of wastewater infrastructure and ongoing operation and maintenance is a necessary component of project planning. Few local governments will pursue such projects if there is no money generated to offset costs. Below are some options for cost recovery:
- Per stall user fees: In the Philippines, at the Alabang Public Market (largest in Metro Manila) in Muntinlupa City, a fee of 5 pesos per day per stall was instituted to pay for the infrastructure and operating costs of their wastewater system that was installed in 2006. The fee has been a great success that resulted in full cost recovery of the treatment system in 3.5 years. After the infrastructure cost is covered, the fee will be used to pay for ongoing operation and maintenance.
- Annual assessment fees: Typically each market stall pays an annual assessment fee. In San Fernando in the Philippines, a 10% assessment increase covers the cost of the wastewater system plus several other improvements. See Example 1 below for more information on San Fernando's system.
- Public toilet fees: At the public market in Dumaguete, also in the Philippines, the public toilet fee is used to fund the wastewater system. For this case, over a million pesos a year is collected from these fees alone, which helps pay the capital and operations costs. Dumaguete's system is described in detail in Example 2 below.
Example 1: San Fernando City, La Union Province, Philippines
The wastewater system for this public market utilizes an upflow anaerobic filter combined with a mechanical aeration system to provide discharge-quality effluent. Due to a lack of space at the market site, a remote location was chosen for the wastewater treatment plant, requiring a network of pressure sewer pipes and a lift station. In order to keep the lift station free of grease and trash, a program of daily cleaning has been implemented. To offset costs, the treatment system is equipped with a filtration and disinfection device that allows for effluent reuse and recycling.
Project Details
Flow: 210 cubic meters per day
Land area: 75 square meters
Number of stalls: 1,315
Treatment Modules
Settling tank
Upflow anaerobic filter
Sequencing batch reactor
Multimedia filter, chlorination
Cost
Treatment plant: 5.4 million Philippine Pesos (P)
Operations: P1,250 per month
Installation cost per stall: P4,106
Example 2: Dumaguete City, Negros Oriental Province, Philippines
The wastewater system for this public market utilizes an anaerobic baffled reactor coupled with a planted gravel filter to treat the wastewater. This is a passive system with no energy input requirements, but it uses considerably more land than mechanical counterparts.
Project Details
Flow: 80 cubic meters per day
Land area: 455 square meters
Number of stalls: 1,255
Treatment Modules
Settling tank
Anaerobic baffled reactor
Anaerobic filter
Planted gravel filter
Polishing pond
Cost
Treatment plant: P3.4 million
Operations: P5,000 per month
Installation cost per stall: P2,709
Conclusion
Wastewater treatment systems for public markets are within the means of most local government units as financing models for full cost recovery . For planning purposes, consider the average cost of a treatment system at between 2,700 and 4,100 pesos per stall, where the higher number is for more mechanized systems and the lower number for passive technologies. For sewer line expenses, add an additional 20% to 25%, or double this if pump stations will be required. Remember, a full engineering study with detailed architectural designs is necessary to determine actual costs.