Improvement of Algae Settleability in High Rate Ponds Using Rotifers at Richmond Field Station

Rotifers offer a natural and efficient means to improve algae settleability in high rate pond systems, a wastewater treatment system, through their selective grazing on smaller algae (1-10 micrometers). However, the survival of the rotifers in these water systems is highly dependent upon pH and ammonia concentrations. Overgrazing on algae results in increased ammonia levels and pH which are toxic and inhibit rotifer growth. An experiment was carried out to study improvement of algae settleability as well as rotifer sensitivities to pH and ammonia, using samples of high rate pond water with different added rotifer concentrations and a control with no rotifers. Ammonia, pH, turbidity, rotifer and algae populations were monitored at four time intervals (0, 2, 4, and 7 days). The results indicated a significant improvement in algae settleability with increased rotifer populations. However, due to the rapid reproduction of rotifers, overpopulation of rotifers resulted in overgrazing on small algae which altered the water characteristics, pH and ammonia. Overpopulated treatments experienced a die off of rotifers caused by excessively high ammonia concentrations which reduced algae settling. Introduction Worldwide water consumption is a growing concern because it has increased eightfold over the past century (Gleick, 1998). Freshwater resources are especially limited because most of it is frozen in ice caps. The remaining freshwater resources, mostly groundwater, are not fully accessible since much of it is too deep to economically exploit (Alvarez 2000). Therefore there is a strong need to protect and conserve our water resources. Wastewater treatment offers a practical solution to resolve our water needs. In 1972, the Clean Water Act passed to provide funding to create effective wastewater treatment methods while setting pollution discharge and primary water treatment standards. In 1977, the act was amended to require secondary treatment of wastewater to include biochemical oxygen demand and total suspended solids to all municipal wastewater before discharge. Many community wastewater plants experienced difficulties in meeting these stringent effluent requirements due to cost, reliability, and maintenance (USEPA 1998). Conventional secondary treatment systems include oxidation ponds, aerated lagoons, trickling filters, and activated sludge systems, with each system having its own limitations such as land requirement, cost, and effluent characteristics. An innovative treatment method, the high rate pond, is cost effective and capable of producing effluent that is low in dissolved organic material, metals, nutrients, and hazardous bacteria that meet secondary treatment requirements. High rate ponds are shallow (20-50 cm) ponds designed in a racetrack configuration and is continuously mixed to reduce land requirement and maximize algae production. High rate ponds rely on the process of photosynthetic oxygenation to treat wastewater (Oswald 1988). Figure 1: Cycle for Photosynthetic Oxygenation In photosynthetic oxygenation, aerobic bacteria and algae share a symbiotic relationship in which bacteria depend on algae for oxygen, while algae depend on bacteria for nutrients. The bacteria oxidize the entering organic wastes to produce carbon dioxide, ammonia, phosphate, and bacterial biomass. The algae fix these organic wastes which are then volatilized or precipitated out of the wastewater (Puskas 1994). Also, the algae remove heavy metals by using its affinity for polyvalent cations to improve settling (Oswald 1988). By converting light energy to heat and by increasing pH, disinfection of coliform bacteria is increased dramatically (Cromar 1996). After filtration to remove algae, high rate ponds have been shown to reduce BOD and suspended solids below required standards. In comparison to conventional treatment methods, high rate ponds require half as much land as oxidation ponds, have shorter residence times and lower evaporation losses, lower aeration costs than mechanical systems, and require little maintenance and no chemical disinfectants (Esen 1991). In addition, high rate ponds produce large amounts of algae that can be used as soil conditioners or fermented to produce methane for energy production (Rovisora 1996). High rate ponds provide a promising wastewater treatment method by reducing bacteria, BOD, and nutrients, however the effluent is rich in small algae (less than 10 micrometers) due to slow separation in the liquid phase. As a result the effluent does not meet water quality criteria on suspended solids. Conventional solids removal techniques are applicable to high rate ponds but are expensive (50-60% of the total cost) and difficult (Esen 1994). For instance, polymer coagulants can be used to neutralize the charge on particles to improve settleability of the smaller algae. Microscreens and sand filters have been developed to remove most of the algae however, throughput rates are too slow and there are problems with clogging due to smaller algae (Esen 1991). Bioflocculation, autoflocculation, and sedimentation are capable of producing effluent that meet secondary standard but are often inconsistent. Due to the ionic charges and small size of algae such as Chlorella and Chlorococcum (2-3 micrometers), they remain suspended in the effluent for long periods of time. Zooplankton grazing offers an alternative solution to improving algae settleability. Studies on rotifers (brachionus calyciflorus) reveal that they selectively graze on smaller algae ranging from 1 to 10 micrometers (Hansson 1998). After digestion, rotifers produce fecal pellets, which are composed of many smaller algae lumped together, called flocs. This drastically reduces suspended solids in wastewater since the fecal pellets are larger and settle significantly faster. Because rotifers are capable of reproducing at rapid rates they often result overgrazing of algae. An initial population of 100 rotifers can increase to 10, 000 in ten days (Hoff 1997). Overgrazing on algae significantly alters water characteristics, specifically pH and ammonia. Previous research shows that rotifer survival and reproduction are extremely sensitive to pH, temperature, and ammonia concentrations (Schluter 1985), however little else is known about these organisms. The purpose of this research project is to assess the improvement of algae settleability in high rate ponds populated by rotifers. Four separate algae cultures were tested with variable concentrations of rotifers: 0, 25, 75, 200 per/mL. Counts of rotifer and algae populations (small and large), and turbidity were recorded in order to confirm selective grazing and evaluate settling improvement. pH, temperature, and ammonia concentrations were measured to determine how the microenvironment changed in response to varying rotifer populations. These water characteristic variables are critical to rotifer reproduction and survival. Therefore, under certain temperature, pH, and ammonia conditions, created by varying rotifer concentrations, selective grazing of rotifers on smaller algae (1-10 micrometers) will improve total algae settleability. Methods This study took place at the Algae Laboratory at the Richmond Field station. Water samples were taken from the Delhi Advanced Integrated Wastewater Pond System located in Central California. The pond has a 1.5 million gallon per day design flow and takes up 22 acres. The wastewater is from municipal sources and is pretreated for large algae using percolation beds (Lundquist 2000). Four 500 mL beakers filled with Delhi pond water were continuously illuminated in incubation chambers at temperatures of 22-24 degrees Celsius. Added to each beaker was a different concentration of rotifers (Brachionus calyciflorus) (0, 25, 75, 200) with one beaker serving as a control (zero rotifers). Monitoring of algae and rotifer populations, pH, temperature, ammonia, and turbidity occurred at 0, 2, 4, and 7 days for each treatment. At the end of each 7 day trial, samples were replaced with fresh Delhi wastewater, which were taken on a weekly basis from the High Rate Pond. A total of ten replicates were performed. The algae and rotifer concentrations was measured using a known volume of a pipette under a high-powered microscope following Copenhagen’s methods, 1998. The rotifers were anesthesized with isopropyl alcohol to facilitate the counting. Dominant algae species were identified and categorized as small (1 to 10 micrometers) and large (greater than 10 micrometers). Temperature and pH was measured in degrees Celsius using a Denver Instruments universal meter. To get the most precise pH values, manual standardization of pH in buffer solutions were performed. Ammonia concentrations were measured using LaMotte brand ammonia test kits. Turbidity was determined using a 6145 Klett meter, standardized with deionized water.