Ecological Process Fundamentals of Ecological Machinery

1. AERATED WATER TREATMENT WETLANDS Aeration overcomes oxygen transfer limitation in existing VSB wetlands, allowing aerobic degradation of BOD and nitrification LAKES – THE PROBLEM Lakes in temperate climates are often dimictic. That is, thermal stratification dissipates in early spring and late fall. Here, we consider eutrophic and certain mesotrophic lakes. 1) A transient episode of complete mixing ensues in this turnover period before thermal stratification resumes. 2) In eutrophic lakes, early spring is often a period of clear water during which photosynthetically active radiation (PAR) can penetrate deeply. 3) The hypolimnion typically maintains a positive dissolved oxygen concentration as long as long as it receives sufficient light to support photosynthesis. 4) As the epilimnion temperature increases, phytoplankton growth eventually shades the hypolimnion, which quickly becomes anoxic until fall turnover. 5) A cascade of ecosystems effects follows this process, resulting in the classic, and culturally undesirable, “pea soup” look of eutrophic lakes in summer. 1 2 & 3 4 & 5 PAR PAR PAR ANOXIC Aeration specific effects on treatment rates and internal treatment processes • 1. Aeration influences • volatilization • oxidation reduction potential • aerobic respiration • internal dispersion • 2. Cycling of aeration sequences allow • aerobic / anoxic / anaerobic shifts in the wetland reactor • THE SOLUTION? • Destratification by mechanical means, especially aeration, has been practiced for over 50 years. • Thermal resistance to destratification can be overcome by mechanical means with very low power inputs in some lakes and ponds. • In others, destratification can be a vexing engineering problem with no easy solution. • Examples: • A forty hectare lake of 7 m average depth with large littoral shallows could be destratified with less than 37 kW (50 hp) of machinery operating at heads of 3 – 5 cm. • This works IF circulation patterns can be established that alter the thermal budget sufficiently to overcome stratification with a weekly pumped lake volume exchange. • Mere mechanical mixing may not be a sufficient energy subsidy as can be seen in an elementary Richardson number analysis. • The Richardson number is the ratio of potential to kinetic energy: • Ri = (ρhypo – ρepi) / ρo (gh/u2), whereρ= water density, g = gravitational constant, h = water column height, and u = mixing velocity. • Ri < 0.25 => spontaneous mixing. Observe how large u must be to cause mixing if h and density (ρ) differences are large. Velocity is seldom sufficient to destratify. • Ri >> 1 => Buoyant forces predominate. Think of the epilimnion in a deep lake as a giant cork. Destratification is a dubious scheme in this case. • A deep, multi-basin lake may not have a reasonable mechanical destratification solution because … • Thermal resistance to mixing (work) is proportional to surface area times the square of water depth. • Work (ergs) = (Alakeh2 / 12)(ρhypo – ρepi) =>Forget mechanical mixing for large, deep lake of lakes, except to establish local, aerated refugia for selected organisms. Three treatment areal rate constants, m/yr for groundwater remediationpilot wetland unit in Casper, Wyoming Full-scale aeration system during startup testing Local mixing only without thermal gain. Mixing delivers water to zones of thermal gain BP Former Refinery, Casper Wyoming. Design flow is 6,000 m3/day (1) Basin depth & morphology compatible with mechanical destratification North American Wetland Engineering - PA (2) Basin depth & morphology incompatible with mechanical destratification Full-scale system has produced non-detect levels of petroleum hydrocarbons since May 2003 commissioning Pond Restorer May work as a refugia design in shallow lake sub-basins. Ecological Process Fundamentals of Ecological Machinery Scott Wallace – North American Wetland Engineering, 20 North Lake Street, Forest Lake, Minnesota  55025 www.nawe-pa.com David Austin – Dharma Living Systems, 8018 NDCBU, 125 La Posta, Taos, New Mexico 87571 www.dharamlivingsystems.com • INTRODUCTION • All ecosystem receive energy inputs from natural sources. • Engineered ecosystems receive energy from mechanical sources in construction, operation or both. • Ecosystem self-organization is controlled by energy inputs. • If operating design goals cannot be met by natural energy inputs then operational machinery must be employed. • Here, we consider process fundamentals of operating ecological machinery. • We offer two brief examples for this poster (this subject really needs a large textbook): • Hydraulic machinery in temperate, shallow, eutrophic lakes to prevent formation of anoxic hypolimnia. • Aeration machinery in treatment wetlands to manage effluent quality. • MECHANICAL MIXING CAN WORK . . . BUT SUSTAINED SUCCESS REQUIRES CAREFUL ECOLOGICAL & SOCIAL DESIGN • Lake restoration is non-trivial. • Mixing design requires coupled hydrodynamic and thermal modeling as a first-order determination of project feasibility. • Light penetration is substantially a function of phytoplankton density. • Mixing may increase light penetration, but only if it is coupled to appropriate ecosystem processes. • Phytoplankton density is a function of nutrient availability and filter-feeding (pelagic & benthic) communities. • Filter-feeding community structure can be radically improved by elimination of hypoxic hypolimnia, but … • Biomanipulation may still be required to control planktivorous fish populations. Public acceptance / cooperation? • Nutrient availability is a complex design / management problem that mixing may or may not address. • Successful destratification may shift primary productivity from algae to submerged macrophytes. Culturally acceptable? • Ecological machinery is just an energy subsidy, not a complete design.