The REOS System: Remote Electro-Optical Sensor

What is the REOS System?

The Remote Electro-Optical Sensor (REOS) system is an automated water quality monitoring system designed to continuously monitor nuisance algae populations in drinking water reservoirs. Using optical instruments originally developed to monitor marine phytoplankton, the REOS system provides reservoir managers with early warning of increasing algae populations.

Planktonic algae are a leading cause of water quality problems in reservoirs. By detecting algal blooms early, reservoir managers can:

• Respond quickly to the taste, odor, and appearance problems that often accompany a phytoplankton bloom
• Reduce the amount of disinfectant needed to control a bloom
• Reduce the associated production of potentially carcinogenic disinfection by-products (DBPs).

How Does REOS Work?

The REOS system is composed of an array of optical and electrode-based instruments deployed on a mooring. The system continuously monitors the optical properties of the water, as well as other water quality parameters, in order to detect the initial stages of an algal bloom. The most important parameters for early bloom detection—chlorophyll concentration and water clarity—are monitored optically with two small, lightweight underwater radiometers (PRR- 600s). The instruments are designed to measure seven wavebands of downwelling irradiance and upwelling radiance as well as water temperature and pressure/depth. These individual wavelengths, ranging from 380 nm to 683 nm, were selected based on ongoing research by the bio-optical sciences community.

Diagram of REOS System

The location of the two instruments is determined based on local reservoir conditions. The radiometers must be located deep enough to avoid backscattering sunlight, but shallow enough to measure the region of most rapid algae growth at maximum sensitivity. At one LADWP reservoir, the PRR-600s are located at a depth of 1 meter and 6 meters.

Each PRR-600 is suspended in a protective stainless steel frame accompanied by a programmable wiper system. The wiper cleans the optical sensors at user-programmed intervals in order to reduce the effects of biofouling.

A third instrument, the DEB-600 (Digital Electrode Bus), is typically located between the two PRR-600s and is used to monitor additional water quality parameters including temperature, pH, conductivity, and oxidation-reduction potential (ORP).

Data are also collected from instruments located onshore. These include a multi-instrument meteorological station and a radiometrically matching radiometer (PRR-610) that measures downwelling irradiance at the surface.

Powered by a Pentium®-based remote acquisition controller (RAC) located onshore, continuous measurements of each parameter are collected, averaged at programmable intervals (e.g., every 15 minutes), and stored in a Microsoft Access® database. Once stored, these data are available to reservoir managers by modem or through a network, and may be viewed in real time or transferred to a remote computer. A central controller can be set up to automatically retrieve the resulting data and print a daily reservoir status report.

How Does REOS Measure Chlorophyll Concentration?

Based on optical data gathered by the PRR-600s, the REOS system estimates chlorophyll concentration using two independent algorithms. Field data collected by reservoir biologists is used to fine tune these algorithms.

At low to moderate concentrations of chlorophyll, the REOS system uses a patented “natural fluorescence” algorithm to calculate chlorophyll, following techniques developed for oceanographic research.

All of the light energy absorbed by plants is not directly converted to chemical energy. Some of this energy is lost as heat and a small but significant portion is re-emitted as fluoresced red light centered at a wavelength of 683 nm. Research on marine phytoplankton over the last decade has shown that this natural fluorescence, stimulated by sunlight, can be used as an integrated measurement of chlorophyll.

The natural fluorescence measurement made by the upwelling radiance sensor on the PRR-600 differs from chlorophyll fluorescence measured using a strobe fluorometer in that the fluorescence signal measured by REOS results from the same natural source that is driving photosynthesis in situ—the sun.

At higher concentrations of chlorophyll, the REOS system uses a multispectral algorithm to calculate chlorophyll concentration. In an approach similar to satellite remote sensing of ocean color, the PRR-600s measure available light in multiple wavebands to determine concentrations of pigments. Attenuation in the blue and red regions of the spectrum by phytoplankton and detritus will cause changes in the relative spectral distribution of light in the water column. From these values, changes in the ratios of the signal from different regions of the spectrum can be empirically related to changes in the constituents of the water column, such as increases in the phytoplankton crop.

REOS wavebands are also used to calculate turbidity equivalents. Turbidity equivalents can be used as a proxy for nephalometric turbidity units (NTU), a primary drinking water standard, and to calculate water transparency.

Why is REOS Useful?

The primary goal of the REOS system is to help water reservoir managers develop effective disinfection or dilution strategies. By continuously monitoring algae levels, reservoir turbidity, and other water quality parameters, the REOS system allows early detection of algal blooms. Early detection allows reservoir managers to interrupt a bloom cycle in the initial stages of exponential growth when the algae population is smaller and more manageable. By controlling blooms with less disinfectant, water managers can also reduce treatment costs.

U.S. Patent No. 5,905,570.

Click here to download a copy of the REOS brochure (338 KB).