Urban Sanitation: New Terminology for Globally Relevant Solutions?

Progress toward Sustainable Development Goals for global access to safe sanitation is lagging significantly. In this Feature, we propose that misleading terminology leads to errors of categorization and hinders progress toward sanitation service provision in urban areas. Binary classifications such as "offsite/onsite" and "sewered/nonsewered" do not capture the need for "transport to treatment" or the complexity of urban sanitation and should be discarded. "Fecal sludge management" is used only in the development context of low- or middle-income countries, implying separate solutions for "poor" or "southern" contexts, which is unhelpful. Terminology alone does not solve problems, but rather than using outdated or "special" terminology, we argue that a robust terminology that is globally relevant across low-, middle-, and upper-income contexts is required to overcome increasingly unhelpful assumptions and stereotypes. The use of accurate, technically robust vocabulary and definitions can improve decisions about management and selection of treatment, promote a circular economy, provide a basis for evidence-based science and technology research, and lead to critical shifts and transformations to set policy goals around truly safely managed sanitation. In this Feature, the three current modes of sanitation are defined, examples of misconceptions based on existing terminology are presented, and a new terminology for collection and conveyance is proposed: (I) fully road transported, (II) source-separated mixed transport, (III) mixed transport, and (IV) fully pipe transported.

Persistence of pathogens and bacterial community dynamics in tropical soil after application of raw sewage.

The objective of this work was to evaluate the persistence of faecal indicators and pathogenic organisms (Salmonella spp., Escherichia coli and viable helminth eggs) and the structure/diversity of bacterial communities in soil receiving raw sewage (RS) for an extended period of application (3 uninterrupted years). In the experimental design, three treatments were defined: (1) Control soil, characterized by the analysis of a composite sample collected in an area of similar soil, but not a recipient of RS (TSC); (2) Soil receiving conventional mineral fertilization, and furrow irrigation with supply water (TW); and (3) Fertirrigated soil with RS applied by furrows (TF). The results of persistence of pathogenic organisms and indicators in TF indicated a sanitary quality similar to the control soil (TSC), thus potentially bringing low risks of contamination with pathogens present in the soil. The presence of viable helminth eggs was not identified in any treatment studied, because of its low concentration in the raw sewage of the studied system. The TW, TF and TSC treatments had 34.8% of bacterial diversity in common. The bacterial composition of the soil showed a predominance of the Proteobacteria phylum in all treatments studied; however, TF was the one with the highest relative abundance of this phylum (44.8%).

Macronutrient distribution in soil subjected to raw sanitary sewage application by closed-end furrows.

This work aimed to evsaluate the influence of raw sanitary sewage (RS) application in closed-end and level-bottom furrows on the distribution of macronutrients and sodium in the soil along its depth. The applied amount of RS was controlled due to the harmful effect of sodium, with a weekly application over three consecutive years. The experimental design consisted of two experimental plots receiving RS, one with alternating the site of effluent application (alternation of inlet and outlet-TFA) and the other without alternating the site of application (only inlet-TFN); and two experimental plots receiving conventional mineral fertilization, both irrigated with public water supply, where one alternated the site of water application (alternation of inlet and outlet-TWA) and the other without alternating the site of application (only inlet-TWN). The inversion of the flow direction with RS application in the closed-end furrows (TFA) provided a better distribution of nutrients in the soil along its length. There was no significant difference in the levels of macronutrients and sodium between the initial and final regions of the furrow soil. In contrast, the fixed flow direction experimental unit (TFN) exhibit a significant difference between the initial and final levels of total nitrogen, phosphorus, potassium and sodium in the closed furrows with mean concentrations of 265.2, 16.2, 46.7 and 110.0 mg dm-3, respectively.

What is the best design approach for estimating effluent concentrations from horizontal flow treatment wetlands: the use of volumetric (kV) or areal (kA) removal rate coefficients?

Effluent concentrations from horizontal flow (HF) treatment wetlands can be estimated by using the Tanks-In-Series model for describing hydraulics and first-order removal rate coefficients for describing pollutant removal. In the design of conventional wastewater treatment plants, volumetric removal rate coefficients (kV) are traditionally used in conjunction with the theoretical hydraulic retention time. Areal removal rate coefficients (kA) coupled with the applied areal hydraulic loading rate are widely used in the literature. Despite this, supporting evidence of its appropriateness is scarce in the literature. The objective of this study is to investigate the adequacy of both approaches by analyzing the influence of liquid depth on kV and kA. Data from 74 HF wetlands were collected, covering biochemical oxygen demand and chemical oxygen demand, and diverse types of influents (raw sewage and primary, secondary and tertiary effluents). For these conditions, kV decreased with depth of the wetland system. Regression analyses between depth and removal rate coefficients were performed, and the equations indicated that kV was approximately related to the inverse of depth, while kA was almost independent of depth. These findings endorse the utilization of the areal-based approach for design purposes. The volumetric-based approach can also be used, but the value of kV must be provided together with the depth being considered.

What is the best procedure for determining removal rate coefficients in horizontal flow treatment wetlands: influent and effluent concentrations or longitudinal concentration profiles?

First-order removal rate coefficients (k) are used in predictive equations for estimating effluent concentrations from horizontal flow (HF) wetlands. Due to limited resources, influent and effluent concentration data from existing systems are frequently used in the estimation of k values from operating systems, but another choice is to use concentration data along the longitudinal profile of the HF wetland. Based on a dataset with 41 HF wetlands/studies obtained from a literature survey, with chemical oxygen demand (COD) measurements at different sampling points, volumetric (kV) and areal (kA) removal rate coefficients for the Tanks-In-Series (TIS) model have been obtained using the two estimation methods. In general, removal rate coefficients derived from longitudinal profiles of concentrations were higher than those obtained by using data from influent and effluent concentrations, reflecting the fact that constituent removal is mostly accomplished before the wastewater reaches the outlet zone. Deriving coefficients from longitudinal profiles is more comprehensive, providing a better explanation of the internal removal taking place in the treatment wetland. However, the more widely used approach of calculating kV and kA from influent/effluent concentrations may lead to a safer design of horizontal flow wetlands, because of underestimation of the actual removal rate coefficients.

Simple mid-depth transverse baffles to improve bacterial disinfection in a shallow maturation pond - performance evaluation and CFD simulation.

Sunlight-mediated disinfection in wastewater treatment ponds is widely recognised as the main role for disinfection, but pathogen removal can be limited by poor light penetration in the water column. The aim of this research is to evaluate a different and simple type of intervention for improving bacterial disinfection, by placing mid-depth transverse baffles perpendicular to the theoretical flow in an existing shallow maturation pond, aiming at increasing the opportunity for bacteria to receive radiation at the upper layers of the pond. In order to understand the hydrodynamic mechanisms and evaluate the effectiveness of the proposed intervention, a 3D Computational Fluid Dynamic (CFD) model was applied, including the transverse baffles and a particle-tracking model. This was complemented by monitoring E. coli in the pond, in the phase before and after inclusion of the transversal baffles. Overall, the transverse baffles increased disinfection efficiency by moulding/re-routing the hydrodynamic flow and increasing microorganism exposure to the sunlight.

Relationships between abiotic and biotic variables in a maturation pond and their influence on E. coli removal.

The effects of depth and climate seasonality on zooplankton, algal biomass, coliforms and Escherichia coli in a small full-scale shallow maturation pond receiving pre-treated domestic wastewater were evaluated during a tropical climatic seasonal cycle. The experiment revealed that the zooplankton community was dominated by rotifers and protozoans, and concentrations were influenced by seasonality. A negative correlation between zooplankton, and pH, dissolved oxygen, temperature and ultraviolet radiation, and chlorophyll-a and Escherichia coli were observed at all depths. The major driving forces influencing Escherichia coli were pH, dissolved oxygen, ultraviolet radiation and the zooplankton. A significant difference between Escherichia coli removal throughout the three different depths were observed. Both bacterial and zooplankton concentrations were greater closer to the bottom of the pond, therefore reinforcing the integral role of solar radiation on bacterial removal. These results give an insight on the dynamics of these groups in pond systems treating domestic wastewater, by correlating the variation of zooplankton with biotic and abiotic variables and seasonal changes in a tropical climate, where few studies have been performed on this topic.

Dynamics of the behaviour of a vertical wetland (French system) operating in warm-climate conditions, evaluated by means of variables continuously measured in situ.

The sewage treatment system in this study was operated with only the first stage of a French system of vertical wetlands, composed of two units in parallel and running with an extended feeding cycle (7 days). This research sought to evaluate and relate continuous variables measured in situ (dissolved oxygen (DO), pH and redox potential) throughout the feeding cycle, with measurements at distinct heights along the filter vertical profile. Additionally, the influence of the surface organic sludge deposit was investigated. A close link between the hydraulic behaviour and the effluent quality was verified, with both being related to the batch volume and the instantaneous hydraulic loading rate. The drop in DO as the feed days progressed could be related to the loss of hydraulic conductivity. A thicker sludge layer decreased the aeration capacity of the filter. The effluent was observed to be aerated when percolating through the medium. DO and pH data suggested that nitrification varied along the filter depth, the batch duration and the feed cycle. The monitored parameters may be indicative of the behaviour of other parameters.

A simple field essay for detecting departures from expected performance in small-scale, remote or rural wastewater treatment plants.

A scientific basis is given to the traditional method of inferring effluent quality based on visualization of samples in transparent flasks. A scale of 1-6, with different printed grey intensities, is placed behind transparent PET bottles containing the sample, and gives an indication of the range of turbidity in the sample (1 is the most transparent and can only be visualized if the effluent is well clarified; in the other spectrum, 6 is the darkest and indicates highly turbid effluents). Turbidity has been correlated with total suspended solids (TSS), particulate biochemical oxygen demand (BOD) and particulate chemical oxygen demand (COD) based on thousands of monitored data collected in the effluent from seven different treatment processes in Brazil: upflow anaerobic sludge blanket (UASB) reactor, trickling filters, activated sludge, horizontal wetland, vertical wetland, polishing ponds and coarse filter after pond. The method is simple and instantaneous, can be used in virtually all places and in every visit of the operator to the remote treatment plant, allows recording of the image in smartphones, does not use any equipment, chemicals or energy, and has been showed to represent well the effluent quality of existing treatment plants. This essay is complementary and does not substitute specific traditional sampling and analysis, but allows easy inference of deterioration of effluent quality.

French vertical flow treatment wetlands in a subtropical climate: Characterization of the organic deposit layer and comparison with systems in France.

This paper presents an advanced characterization of the organic fraction of the top deposit layer collected in a French vertical flow treatment wetland (first stage) in operation in Brazil (two units with different organic deposit layer accumulation times), and compares the results with those obtained from studies in France. The organic and inorganic constituents of the samples collected were analysed by biological, chemical and thermochemical methods. The unit with the organic deposit with longer accumulation time (almost 10 years) was characterized as a mature one (organic matter - OM: 51.3 and 52.7%, thermal index - RTGA: 0.79 and 0.85, humification index - HI: 0.61 and 1.16, respectively for the depths of 0-5 cm and 5-10 cm). The unit with the deposit organic with less than three years of accumulation also presented characteristics of a mature deposit (OM: 61.2%, RTGA: 0.79, HI: 1.01 for the depth of 0-5 cm), indicating a rapid mineralization of the deposit under the existing subtropical environment. Despite several differences in term of conception, hydraulic and mass loading rates and accumulation rates, the characteristics of the OM of Brazilian deposits were found to be quite similar to the French ones.

Reduction of area and influence of the deposit layer in the first stage of a full-scale French system of vertical flow constructed wetlands in a tropical area.

Utilization of the French system of vertical wetlands for treating raw sewage keeps increasing, but there is still limited consolidated information on their long term use in tropical countries. Under these conditions, there are indications that surface area requirements can decrease, whilst still keeping a satisfactory performance. However, variations in the operational mode and the role of the surface organic deposit layer under warm climatic conditions have not been fully investigated. The goal of this work was to evaluate the performance of a system comprised of only the first stage of the French system, with a further reduction of 1/3 of the area (utilization of only two units in parallel, instead of three) in terms of organic matter removal and nitrogen conversion, with one unit with a deposit layer accumulated over 9 years of operation, and the other unit without sludge layer, under Brazilian tropical conditions. The system was originally designed according to Cemagref/Irstea recommendations for the first-stage of the French system for the treatment of raw sewage generated by an equivalent population of 100 inhabitants. However, it was later on changed, and operated with only two units, using only 0.6 m2·pe-1. Feeding and resting periods were of 7 days each. In order to evaluate the influence of the sludge layer, the top sludge from one of the units was removed, and the performance of both units was compared by the Mann-Whitney test. The database comprises the wetland performance values in terms of dissolved oxygen (DO), redox potential (Eh), chemical oxygen demand (COD), biochemical oxygen demand (BOD), total suspended solids (TSS), total Kjeldahl nitrogen (TKN), NH4 +-N and NO3 +-N, covering a monitoring period of 15 months. The effluent DO concentrations were significantly lower in the unit with top sludge, but still sufficiently high in both units. Although there were some variations between both units, effluent concentrations of the major pollutants were not significantly different in the units with and without sludge, and removal efficiencies based on mean values were considered good, given the reduced area of the system: BOD (80% and 79%), TSS (85% and 82%), TKN (60% and 63%) for the units with and without sludge, respectively. Under Brazilian climatic conditions, with the reduced area and employing longer feeding cycles (7 days), the sludge accumulation rate (less than 1 cm·year-1) was lower compared to the French mean values.

From start-up to heavy clogging: performance evaluation of horizontal subsurface flow constructed wetlands during 10 years of operation.

The objective of this work is to evaluate the performance of two horizontal subsurface flow constructed wetlands, one planted with cattail (Typha latifolia) and the other unplanted. The distinguishing feature of this study is that it spans a period of more than 10 years, from start-up to a final operation with heavy clogging and full overland flow. For most of the time, starting in June 2007, the system received municipal sewage previously treated in an upflow anaerobic sludge blanket (UASB) reactor, but for one specific period, the pre-treatment was comprised of the UASB reactor and a trickling filter in series. The two constructed wetlands worked in parallel, each serving approximately 50 p.e. and continuously receiving a flow around 7.5 m3 d-1 for most of the time. The beds had a length of 25 m and a width of 3 m and were filled with blast furnace slag. For most of this long operational period, performance was very good in terms of biochemical oxygen demand (BOD), chemical oxygen demand (COD) and total suspended solids (TSS), with median effluent concentrations of 19/18, 46/52 and 12/8, respectively (planted/unplanted units). Clogging was noticeable in the first years of operation, soon leading to overland flow. However, treatment performance was still successful, even when the system's hydraulics were strongly deteriorated. The type of pre-treatment and the applied loads influenced more the performance of the units than the period of operation itself, evidencing the robustness of the system.

Outflow dynamics in a French system of vertical wetlands operating with an extended feeding cycle.

The possibility of using the first stage of the French System (FS) of vertical wetlands composed of only two units in parallel requires hydraulic investigations to allow a better understanding of its operation under tropical climatic environments. This study evaluated the pattern of the outflow hydrograph along an extended cycle of operation (seven days of feeding) and the influence of the sludge deposit, rainfall occurrence and duration of pulse application on the outflow hydrograph in a modified full-scale FS in Brazil. The results indicated that, as the feeding cycle days increased, there was an increase in the time of filtration and the internal storage of the liquid volume, probably due to a reduction in the filter permeability. Greater hydraulic gradient favoured the infiltration velocity, decreased the amount of liquid stored within the system, and delayed the loss of permeability. The sludge layer contributed to a momentary liquid retention, and also allowed greater evapotranspiration, reducing the liquid volume to be treated. The sludge deposit seemed to hinder the liquid percolation, especially at the end of the cycle, modifying the hydraulic conductivity of the filter as a whole. Intense rainfall events demonstrated that precipitation could modify the flow dynamics within the system.

Performance of a French system of vertical flow wetlands (first stage) operating with an extended feeding cycle.

The aim of this work was to evaluate the treatment performance in the first stage of a vertical flow constructed wetland - French system (VCW-FS) over an extended feeding period (seven days), in two parallel units, for a population equivalent (p.e.) around 100 inhabitants (total of 0.6 m²·p.e.-1), under Brazilian tropical climatic conditions. One of the units had a greater surface sludge deposit layer, accumulated over nine years of operation, while the other unit had its sludge removed prior to the experiments. Four intensive monitoring campaigns covering all days of the feeding cycle were undertaken and the results were compared with those obtained from the conventional monitoring. The results indicated that, over the days of the feeding cycle, dissolved oxygen concentrations decreased, but were still kept at sufficiently high values for the removal of organic matter. Therefore, chemical oxygen demand (COD) removal, although not high, remained acceptable for compliance with local discharge standards during the whole the period. The NH4 +-N removal efficiency and NO3 --N production were higher at the beginning of the feeding cycle, as a result of the more well-established aerobic conditions, with the nitrification rate decreasing from the third day of feeding. The sludge deposit seemed to hinder liquid percolation, especially at the end of the feeding cycle, thus affecting oxygen transfer. Due to the variability of the results over the feeding cycle, if sampling is to be done once a week, it is important to identify the sampling day that best represents the system's performance.

Colmatação e desempenho de sistemas alagados construídos de escoamento horizontal subsuperficial ao longo de oito anos de operação / Clogging and performance of horizontal subsurface flow constructed wetlands over eight years of operation

RESUMO O presente trabalho objetivou avaliar as condições de funcionamento de dois sistemas alagados construídos de escoamento horizontal subsuperficial (SACs-EHSS), um plantado (SACP) com taboa (Typha latifolia) e outro mantido sem plantas (SACNP), ao longo de oito anos de operação, no tratamento de esgoto sanitário. Mediu-se a extensão do escoamento superficial, como indicativo da colmatação do meio poroso ao longo do tempo, além de diversas variáveis que possibilitem inferências sobre as possíveis perdas de eficiência do sistema na remoção de poluentes dessa água residuária. As avaliações indicaram que o meio poroso dos dois SACs se encontra colmatado (mais de 80% do leito apresentando escoamento superficial), estando o SACNP, atualmente, em pior situação, tendo havido prejuízo do seu potencial de remoção de sólidos e nitrogênio em relação ao SACP. Apesar disso, o desempenho em ambos os sistemas permanece satisfatório, não existindo tendência de decréscimo acentuado nas suas capacidades de remoção de poluentes, contrariando a expectativa e corroborando resultados obtidos em trabalhos de monitoramento desses sistemas por longos períodos de tempo. A carga aplicada e o tipo de pré-tratamento a que o esgoto foi submetido parecem ter sido mais decisivos na eficiência dos sistemas em remover nutrientes/poluentes do que o próprio período de operação das unidades, sobretudo no que se refere ao SACP.

ABSTRACT The objective of this work was to evaluate the conditions of operation of two horizontal subsurface flow constructed wetlands (HSSF-CW), one planted (PCW) with cattail (Typha latifolia), and another, maintained without plants (NPCW), along eight years of operation in sewage treatment. The extent of the overland surface flow was measured as an indicative of the clogging of the pore medium over time, besides several variables that allow to infer possible losses of treatment efficiency in the pollutants removal from wastewater. Evaluations indicated that the porous medium of the two CWs is clogged (more than 80% of the bed with overland surface flow), and that the NPCW is currently in a worse condition, with a reduction of its solids and nitrogen removal potential in relation to the PCW. Despite this, the performance in both systems remains satisfactory, and there is no tendency of decrease in the capacity to remove pollutants, contrary to expectations, and corroborating the results obtained in works that monitored these systems for long periods of time. The load applied and the type of pretreatment to which the influent was submitted seem to have been more decisive in the systems' efficiency in removing nutrients/pollutants than the operating period of the unit, especially for the PCW.

Performance of the first stage of the French system of vertical flow constructed wetlands with only two units in parallel: influence of pulse time and instantaneous hydraulic loading rate.

The technology of vertical flow constructed wetlands - French system for treating raw wastewater depends on several hydraulic factors, one of them being the duration of the pulse feeding and the resulting instantaneous hydraulic loading rate. This paper analyses two scenarios in the same system, the first of a faster feeding by pump and the second of a slower feeding by siphon, both with instantaneous hydraulic loading rate values lower than the literature recommendations. The system treated raw wastewater from a population equivalent of 100 p.e. in Brazil, and was comprised by only the first stage and two units in parallel. The shorter duration of feeding time and higher instantaneous hydraulic loading rate were associated with significantly higher chemical oxygen demand and total Kjeldahl nitrogen removal efficiencies, but with no significant differences in terms of biochemical oxygen demand (BOD) and suspended solids (SS). Oxygen concentrations and redox potential in the effluent were evaluated, together with the effluent flow rate profiles. The removal efficiencies were associated with the accumulation of solids in the upper part of the filter resulting from seven years of operation and to the operating hydraulic conditions, which are important elements in the performance of the system.

Vertical profiling and modelling of Escherichia coli decay in a shallow maturation pond operating in a tropical climate.

Maturation ponds are excellent natural treatment systems for disinfecting domestic wastewater from pathogenic bacteria because of their great sunlight exposure through depth and high pH and dissolved oxygen values - all considered important factors. Escherichia coli (E. coli) removal is higher at closer to the surface of the pond and proceeds to decrease due to the limited amount of sunlight at deeper depths. To quantify the decay coefficients (K b) of E. coli in a shallow maturation pond, quartz vessels were used to create isolated batch experiments and submerged at different depths inside the pond during different periods of the day (morning and afternoon). K b values ranged from 0.48 to 0.28 h-1, considering different depths and both periods. A dark control vessel was also used to quantify dark decay rates (K d) at the same depth and periods as the quartz vessels, and ranged from 0.14 to 0.04 h-1. Environmental variables did not appear to influence overall disinfection. Two models for estimating final E. coli concentrations were proposed, considering the kinetic coefficients obtained in the batch experiments and the dispersed flow regime for continuous flow ponds, which resulted in very good fittings with monitoring data and demonstrated the vertical profiling of E. coli concentration.

Difficulties and modifications in the use of available methods for hydraulic conductivity measurements in highly clogged horizontal subsurface flow constructed wetlands.

Despite the fact that several authors consider the available measurement methods of hydraulic conductivity (ks) suitable for a good representation of the bed condition and clogging potential in horizontal subsurface flow constructed wetlands, others have questioned their adequacy. In this work, hydraulic conductivity measurements with conventional and modified methods were undertaken in two small full-scale units, one planted with cattail (Typha latifolia) and the other unplanted. Both units had already been operating for seven years and showed a high degree of clogging. It was observed that the use of the falling head method, with the introduction of the tubes during the test, provided results without a clear spatial trend. On the other hand, tests done on monitoring wells inserted during construction time showed, as expected, ks increasing with the horizontal distance from the inlet, but without reflecting actual field conditions. It was observed that, as the bed became more clogged, the use of the reported methods became more complex, suggesting the need of other methodologies. The use of planted fixed reactors (removable baskets installed in the bed) with evaluation of ks at constant head in the laboratory showed potential for the characterization of the hydrodynamic properties of the porous medium.

Performance evaluation of a large sewage treatment plant in Brazil, consisting of an upflow anaerobic sludge blanket reactor followed by activated sludge.

The objective of this study was to evaluate the behaviour of a system comprising an upflow anaerobic sludge blanket reactor followed by activated sludge to treat domestic sewage. The Betim Central sewage treatment plant, Brazil, was designed to treat a mean influent flow of 514 L/s. The study consisted of statistical treatment of monitoring data from the treatment plant covering a period of 4 years. This work presents the concentrations and removal efficiencies of the main constituents in each stage of the treatment process, and a mass balance of chemical oxygen demand (COD) and nitrogen. The results highlight the good overall performance of the system, with high mean removal efficiencies: BOD (biochemical oxygen demand) (94%), COD (91%), ammonia (72%) and total suspended solids (92%). As expected, this system was not effective for the removal of nutrients, since it was not designed for this purpose. The removal of Escherichia coli (99.83%) was higher than expected. There was no apparent influence of operational and design parameters on the effluent quality in terms of organic matter removal, with the exceptions of the BOD load upstream of the aeration tank and the sludge age in the unit. Results suggest that this system is well suited for the treatment of domestic sewage.

Solar radiation (PAR, UV-A, UV-B) penetration in a shallow maturation pond operating in a tropical climate.

Solar radiation is considered the primary route for disinfection of pathogenic bacteria in maturation ponds. There is scarce information on depth profiling and attenuation of photosynthetically active radiation (PAR), UV-A and UV-B in shallow maturation ponds operating in tropical climates. Measurements of solar irradiance of the three wavelength ranges, together with turbidity, have been acquired from different depths for over 1 year in a shallow maturation pond (44 cm of depth) operating in Brazil. UV-A and UV-B were still detected at 10 cm from the surface, but from 15 cm both were undetectable. PAR was still detected at 30 cm of depth. Irradiation attenuation showed to be related to turbidity. Attenuation coefficients were calculated and simple models without turbidity (traditional structure) or including log10 of turbidity are proposed for predicting PAR irradiance attenuation as a function of depth.