The Optimization of A Wastewater Stabilization Lagoon System
By: Claudio Arato, B.Sc., P.Eng., Nelson Environmental Inc., 310-1140 Pendrell St., Vancouver, BC V6E 1L4;
and: Martin Hildebrand, P.Eng., Nelson Environmental Inc., 101 Dawson Rd., Winnipeg, MB R2J 0S6
ABSTRACT
This paper presents a case study of the wastewater treatment system for the Village of Hamel, IL, USA. The wastewater treatment system was originally constructed in 1930 as a single cell passive lagoon. In 1975, Hinde Engineering (Lake Bluff, IL) fine bubble aeration equipment and concrete barrier walls were installed to improve effluent quality and increase capacity by delineating the single cell into a three-cell system with a total retention time of 42 days. In 1997 the aeration system was upgraded with an Air Diffusion Systems (ADS) fine bubble aeration. One of two in-ground sand filters was converted to an aerated rock biofilter (living rock filter) to enhance final polishing and ammonia removal. Preformulated natural bacteria and enzymes were applied to solubilize and reduce the approximate 0.45 meter (18 inch) sludge layer online without necessitating removal of the system from service or applying for desludging permits.
Since the 1997 upgrade the system has consistently provided 90%+ removal for biochemical oxygen demand (BOD5), total suspended solids (TSS) and ammonia, all without chemicals and odors. The bacterial augmentation program resulted in a sludge depth decrease of 80 to 90% throughout a three (3) month period in Fall/Winter 1997.
The process upgrades have allowed the Village of Hamel to cost-effectively increase its system capacity, improve the effluent quality to meet current and future discharge standards while realizing significant capital cost savings from not having to physically remove and dispose accumulated sludge. Based on the current population growth, the Village of Hamel has added 15 to 20 years to the life of the system without necessitating significant infrastructure changes.
ADS CONCEPT
The fine bubble aeration technology that was implemented originally in 1977, and then again for the process upgrade and retrofit in 1997, was a successive application of a technology designed by Air Diffusion Systems A John Hinde Company (Lake Bluff, IL). Treatment occurs via a chemical-free fine bubble aeration process simulating the natural mixing and aeration mechanisms that allow rivers to naturally regenerate themselves after a pollutant is introduced upstream.
High density polyethylene (HDPE) headers and diffuser piping are used to convey blower supplied air through slits in the diffuser piping to produce small, 2.5 millimeter (0.1 inch) diameter bubbles introduced at laminar flows to maximize:
- intimate oxygen exchange to eliminate "dead spots" (low dissolved oxygen sections producing anoxic, anaerobic or facultative conditions) by maintaining minimum dissolved oxygen concentrations of greater than 2.0 milligrams per litre.
- the oxidative degradation reactions of the wastewater organic contaminants, as measured by BOD5 and COD.
- fecal coliform and pathogen removal.
- nutrient utilization to minimize organic sludge accumulation.
- mineralization of trace metals.
CASE STUDY: INTRODUCTION
The Village of Hamel, IL was initially established in 1818. Until 1930, the village did not have sewage treatment or collection of any kind. In 1930, a large earthen pit was dug that was intended to be able to store the sewage produced by the village's 250 residents for a period of 120 to 180 days, depending upon the season. The single lagoon was originally sized at 10,100 square metres by 1.2 metres deep (2.5 acres by 4 feet deep) or a volume of approximately 12,000 cubic metres (3.25 million gallons (US)). The settled solids were removed and land applied at a predicted nominal application rate of 11 to 33 kilograms per hectare per day (10 to 30 pounds per acre per day).
Evaluations of the lagoon geometry and sizing indicate that the original design likely utilized a standard rule-of-thumb application of 0.07 kilograms of BOD5 per day (approximately 0.17 pounds of BOD5 per day). Thus the resulting design hydraulic loading of 19 kilograms per hectare per day (17 pounds per acre per day) allowed the passive system to provide adequate treatment for many years, but requiring comprehensive desludging every 5 to 10 years. As the village and the necessary land application frequency of the biosolids grew, the Village of Hamel began to make modifications to the system to improve its overall operability and treatment performance in advance of tightening effluent discharge standards. To this end, two parallel (2) intermittent sand filters (ISFs) measuring 12 by 15 metres (40 by 50 feet) were constructed, with chlorine applied directly to the discharge outfall to a local creek to maintain a residual chloride level of 2 milligrams per litre.
CASE STUDY: PROCESS UPGRADES
Initial System Modifications (1977-1997)
In 1977, the existing single lagoon was reconfigured to a three-cell system without any significant change to the total system treatment volume of 12,000 cubic metres (3.25 million gallons (US)). The modified three-cell lagoon system was equipped with a Hinde Engineering (Lake Bluff, IL) fine bubble aeration system with polyvinyl chloride (PVC) headers and aeration piping to treat the full daily sewage flow of approximately 378 cubic metres (100,000 gallons (US)) with a hydraulic retention time of approximately 33 days. The parallel ISFs were not modified within the scope of this process upgrade project. The village's discharge permit requires a maximum BOD5 of 30 milligrams per litre and a total suspended solids (TSS) concentration of less than 38 milligrams per litre.
The first two cells had operating depths of 2.3 metres (7.5 feet) while the final polishing cell was kept at 1.2 metres (4 feet). Two concrete baffles divided the polished effluent flow to the ISF system. To facilitate improved biological growth, a recycle stream of 10 to 20% of the flow prior to the sand filter and chlorination unit operations was returned to the primary cell.
Despite the village's population growth to almost 1,000 by 1997, as well as the removal of chlorine disinfection in the mid-1980s, the Hinde Engineering fine bubble air diffusion system did not produce a single discharge violation from 1978 to 1997. Sludge accumulation was sufficiently limited to not impact system performance and thus was not required until 1997, when other process modifications and upgrades were completed.
System Upgrade Modifications (1997-1998)
In September 1997, the system required an upgrade and review to assess the long-term viability of the three-cell lagoon and ISF system and its ability to continue the high level of treatment for an increasing population.
The project scope focussed on the physical degradation of the PVC headers and piping as well as the growing depth of settled sludge in each of the three cells. The system had been installed with sufficient capacity to accommodate sludge accumulation of 0.038 metre per year (1.5 inch per year). Actual accumulation was closer to 0.025 metre per year (1 inch per year). The PVC diffuser lines were not substantively buried in the settled sludge layer.
System performance had degraded to the point where the Village of Hamel was risking violating its State of Illinois discharge permit as process inefficiencies had produced sufficient sludge accumulations to impact treatment performance. While the process upgrades resolved the requirement of improving performance, the Village of Hamel had made it clear that it did not wish to incur the sizable costs of desludging each cell, estimated at a total cost of $70,000 to $140,000 (US).
To avoid the costs of desludging, including the permitting process for removing and dewatering sludge, a detailed biological augmentation program was implemented. A realistic but effective plan for the addition of proprietary cultures designed to biologically degrade and solubilize the organic fraction of the settled sludge within each lagoon cell was developed and implemented. This plan produced excellent results at a 2-year cost of approximately $7,500, including labor. The addition schedule was discontinued from November to March.
CASE STUDY: MAINTENANCE
At the writing of this paper, the process upgrade has continued to perform well. Maintenance costs to the Village of Hamel are limited to ensuring regular maintenance is completed on the blower system and the HDPE aeration lines are producing a suitable bubble pattern through the practice of periodic applications of anhydrous hydrochloric acid gas.
CONCLUSION
The Village of Hamel case is a valid model for hundreds of inadequate wastewater stabilization ponds in existence throughout North America. Many of these systems are already 20 to 30 years old and have reached and exceeded design capacity. Towns and villages have expanded around their systems and odors are no longer acceptable. Effluent standards are continually becoming more stringent while traditional package plant system costs proposed to meet these standards are increasingly unfeasible for small to medium-sized communities. The effects of utilizing and optimizing existing wastewater stabilization lagoons with high efficiency, low capital technologies provide these communities with an important opportunity to achieve significant capital and operating cost savings while providing sound environmental stewardship for its users and taxpayers.
Optimization of shallow wastewater and sewage stabilization lagoons with Air Diffusion Systems high efficiency fine bubble aeration, aerated rock filters, sand filters and bacterial augmentation programs can increase system capacity, facilitate organic sludge reduction, control odors, and produce overall improved effluent quality.
AUTHORS
Claudio Arato graduated from the University of BC in 1991 in Chemical Engineering. He has accumulated consulting and process design and operations experience in closed loop and effluent recycle systems in Canada, Australia, Asia and Mexico. He is Senior Process Engineer for Nelson Environmental.
Martin Hildebrand graduated in 1995 from the University of Manitoba in Civil Engineering. He has extensive civil and water works construction and management experience in Canada and Mexico. He is the Manager of Nelson Environmental.