Disinfection by-products in filter backwash water: implications to water quality in recycle designs.

The overall purpose of this research was to investigate disinfection by-product (DBP) concentrations and formation potential in filter backwash water (FBWW) and evaluate at bench-scale the potential impact of untreated FBWW recycle on water quality in conventional drinking water treatment. Two chlorinated organic compound groups of DBPs currently regulated in North America were evaluated, specifically trihalomethanes (THMs) and haloacetic acids (HAAs). FBWW samples were collected from four conventional filtration water treatment plants (WTP) in Nova Scotia, Canada, in three separate sampling and plant audit campaigns. THM and HAA formation potential tests demonstrated that the particulate organic material contained within FBWW is available for reaction with chlorine to form DBPs. The results of the study found higher concentrations of TTHMs and HAA9s in FBWW samples from two of the plants that target a higher free chlorine residual in the wash water used to clean the filters (e.g., clearwell) compared to the other two plants that target a lower clear well free chlorine residual concentration. Bench-scale experiments showed that FBWW storage time and conditions can impact TTHM concentrations in these waste streams, suggesting that optimization opportunities exist to reduce TTHM concentrations in FBWW recycle streams prior to blending with raw water. However, mass balance calculations demonstrated that FBWW recycle practice by blending 10% untreated FBWW with raw water prior to coagulation did not impact DBP concentrations introduced to the rapid mix stage of a plant's treatment train.

Ocean optics phase-function inverse equations.

Equations are obtained for determining the Legendre polynomial expansion coefficients of the phase function from angular or angular-spatial integrals of the radiance measured either in water or at the surface.

Isotropic spherical source analysis for ocean optics.

Plane-to-point transformations are used to develop a version of the Hydrolight computer program with which to compute the spatial dependence of the irradiance and the scalar irradiance of the light field away from an isotropic point source deep within a spatially uniform ocean. The transformations are also used to derive analytic approximations for determining the diffuse attenuation coefficient and the mean cosine of the radiance far from an isotropic point source. Approximations for determining the asymptotic diffuse attenuation coefficient from measurements at only two distances far from the source are derived and numerically tested with the modified version of the Hydrolight computer program. New spatial integrals of the outward irradiance are also derived that provide a different way for correlating the inherent optical properties of seawater.

Design of an integrating cavity absorption meter.

The design of integrating cavity absorption meters of general geometry is analyzed for cases in which the incident illumination of the cavity is spatially uniform and isotropic, such as the meter of Fry et al. [Appl. Opt. 31, 2055 (1992)]. The analysis by Kirk [Appl. Opt. 34, 4397 (1995)] for the probability of photon survival in a spherical meter is extended to general geometries. An estimate of the effect of the shape of the cavity on the estimated absorption coefficient is given.

Algorithms for ocean-bottom albedo determination from in-water natural-light measurements.

A method for determining the ocean-bottom optical albedo R(b) from in-water upward and downward irradiance measurements at a shallow site is presented, tested, and compared with a more familiar approach that requires additional measurements at a nearby deep-water site. Also presented are two new algorithms for estimating R(b) from measurements of the downward irradiance and vertically upward radiance. All methods performed well in numerical situations at depths at which the influence of the bottom on the light field was significant.

Ocean inherent optical property determination from in-water light field measurements.

An algorithm is described and evaluated for determining the absorption and backscattering coefficients a(z) and bb(z) from measurements of the nadir-viewing radiance Lu(z) and downward irradiance Ed(z). The method, derived from radiative transfer theory, is similar to a previously proposed one for Eu(z) and Ed(z)and both methods are demonstrated with numerical simulations and field data. Numerical simulations and a sensitivity analysis show that good estimates of a(z) and bb(z) can be obtained if the assumed scattering phase function is approximately correct. In an experiment in Long Island Sound, estimates of a(z) derived with these methods agreed well with those obtained from an in situ reflecting tube instrument.

Ocean optical source estimation with widely spaced irradiance measurements.

A method was developed for determining the spatial distribution of a source from downward and upward irradiance measurements at a single wavelength in seawater of known optical properties. The algorithm uses measurements at two depths located an arbitrary distance apart and solves two nonlinear equations for two parameters that fit a globally exponential or linear source shape. Complex spatially dependent source shapes can be estimated from an irradiance profile by piecing together estimates from neighboring measurement pairs. Numerical tests illustrate the sensitivity of the algorithm to depth, measurement spacing, chlorophyll concentration, sensor noise, and uncertainty in the a priori assumed inherent optical properties. The algorithm works well with widely spaced measurements, moderate sensor noise, and uncertainties in the optical properties regardless of whether the assumed and true profiles are the same shape.

Ocean inherent optical property estimation from irradiances.

A method is evaluated for estimating the absorption coefficient a and the backscattering coefficient b(b) from measurements of the upward and downward irradiances E(u)(z) and E(d)(z). With this method, the reflectance ratio R(z) and the downward diffuse attenuation coefficient K(d)(z) obtained from E(u)(z) and E(d)(z) are used to estimate the inherent optical properties R(infinity) and K(infinity) that are the asymptotic values of R(z) and K(d)(z), respectively. For an assumed scattering phase function beta , there are unique correlations between the values of R(infinity) and K(infinity) and those of a and b(b) that can be derived from the radiative transfer equation. Good estimates of a and the Gordon parameter G = b(b)/(a + b(b)) can be obtained from R(infinity) and K(infinity) if the true scattering phase function is not greatly different from the assumed function. The method works best in deep, homogeneous waters, but can be applied to some cases of stratified waters. To improve performance in shallow waters where bottom effects are important, the deep- and shallow-measurement reflectance models also are developed.

Ocean optical-property estimation with the Zaneveld-Wells algorithm.

The Zaneveld-Wells algorithm for calculating N inherent optical expansion coefficients from N + 1 measured angle-integrated moments of the radiant light field is investigated. Because the algorithm is well conditioned but sensitive to errors in the spatial derivatives, different approximations for the spatial derivatives are considered. The effects of noise and sensor error on the performance of the algorithm have been evaluated analytically, and testing with randomly sampled simulated noise was performed to assess the stability and sensitivity of the algorithm. Results show that the algorithm is fairly insensitive to sensor noise, but neither using a higher-order finite-difference approximation for the derivatives nor reformulating the algorithm into an integral form was successful in overcoming the large errors observed.

Radiative transfer two-stream shape factors for ocean optics.

The mean upward-scattering coefficient of the downward-traveling photons and the mean downward-scattering coefficient of the upward-traveling photons are two factors needed for the two-stream approximation to the radiative-transfer equation. Numerical values of each shape factor just beneath the surface and at asymptotic depths give an indication of the range of values at intermediate depths in spatially uniform waters with no sources and are used to obtain an approximate depth-dependent model for each shape factor. The shape factors are computed for different surface-illumination conditions, wavelengths, and chlorophyll concentrations.

Ocean source and optical property estimation from explicit and implicit algorithms.

We solve an inverse problem of ocean optics for estimating spatially dependent absorption and scattering coefficients and for determining sources such as fluorescence, bioluminescence, or Raman scattering. The solution requires in situ measurement of the downward and upward plane irradiances and scalar irradiances and a priori estimation of the angular shape of the volume scattering function. Both an explicit algorithm and an implicit one are developed from new two-stream radiative-transfer equations that utilize an asymptotic radiance approximation to close the set of equations. A comparison of numerical tests for the two algorithms is given.

Ocean optics attenuation coefficients: local versus spatially averaged.

The local diffuse attenuation coefficients for monochromatic net irradiance and monochromatic scalar irradiance differ from the corresponding spatially averaged coefficients except when both equal an eigenvalue of the radiative transfer equation. Explicit equations to relate spatially averaged to local coefficients that may be useful when inferring self-consistency (i.e., closure) of optical properties are given.

Bioluminescence estimation from ocean in situ irradiances.

An algorithm is developed for estimating the spatial location and magnitude of a bioluminescent radiation source from measurements of the in situ irradiance and scalar irradiance at two depths. The algorithm is based on the principle of photon conservation. The most direct application of the algorithm requires that the absorption coefficient be known, but the algorithm is useful even if that coefficient is unknown. Numerical tests and an error analysis have been done to test the algorithm numerically. In addition we show that if the estimated source magnitude is nearly constant, that value can be used to estimate the vertical attenuation coefficient of the radiation field.

Multiple scattering corrections for lidar detection of obscured objects.

The importance of multiple scattering for lidar detection of a spherical object obscured by an aerosol is assessed using Monte Carlo radiative transfer calculations. Multiple scattering correction factors are significant and depend on the location and size of the object, and the field of view and time resolution of the detector.

Radiative transfer calculations for characterizing obscured surfaces using time-dependent backscattered pulses.

Numerical calculations for estimating the albedo of a surface and its distance behind an obscuring multiplescattering atmosphere are performed for the idealized case of an instantaneous pulse uniformly illuminating a plane-geometry atmosphere of known properties. This inverse problem is complicated by the fact that the backscattered irradiance is broadened in time because of purely geometric effects arising from the geometry considered; the broadening increases with an increasing thickness of the obscuring medium and if the medium scatters in a more isotropic manner, such as with Rayleigh scattering. Isocline maps of different observables suggest that graphical inversion maps might be useful for some applications of this type.

Seawater optical property estimation from in situ irradiance measurements.

A simple algorithm that solves the inverse problem is numerically tested for estimating the ratio of the absorption to the transport coefficients from measurements of the irradiance and scalar irradiance at depths sufficiently beneath the surface of seawater. Results of radiative transfer calculations with a phase function for San Diego harbor seawater were used to test the algorithm and show that the ratio is quite insensitive to the depth of the measurements and to the direction of incident solar radiation unless the absorption is very weak. A second algorithm, based on the assumption that the radiance in the diffusion regime is proportional to a (one-parameter) Henyey-Greenstein shape, is examined for estimating the ratio of the backward scattering to absorption coefficients from only irradiance measurements. The algorithm has the advantage that no scalar irradiance measurements are required, but it provides significantly poorer estimates because the radiance is not sufficiently close to the Henyey-Greenst in shape.

Transport calculations for light scattering in blood.

In vivo measurement of the oxygen saturation levels in blood may be obtained from relative amounts of backscattered monochromatic light at two different wavelengths, as measured with a fiber-optic catheter oximeter. Because of the short mean free path length of light in blood, the backscattering can be well approximated by a previously-derived, one-wavelength transport theory solution for the half-space searchlight problem. This solution, unlike simple diffusion approximations has the advantage that the boundary condition describing illumination of a localized area of blood by a monodirectional light beam can be rigorously satisfied. Sample calculations using the solution are compared with experimental values of the reflectance of blood.