Overflow Rate Research Articles

Estimating trap efficiency of small reservoirs and ponds: methods and implications for the assessment of sediment yield

Throughout the world, several millions of small ponds exist for water supply, irrigation, flood control or to control water quality downstream. The reduced flow velocity in these ponds causes sedimentation of tranported particles. For most ponds this is a negative impact as their retention capacity decreases due to sedimentation processes. Sediment volumes in small ponds can be used to reconstruct sediment yield values and to study the spatial variation in sediment yield over large areas. Especially in developing countries, this technique can be very helpful in establishing large data sets on sediment delivery as there are often no resources for expensive monitoring programmes. However, when such studies are undertaken, one has to take into account the efficiency of the pond in trapping sediments. This trap efficiency is dependent on the characteristics of the inflowing sediment and the retention time of the water in the pond, which in turn are controlled by pond geometry and runoff characteristics. Because trap efficiency is one of the most important properties of a pond or reservoir, it has been studied for quite some time. This article provides an overview of the different methods available to estimate the trap efficiency of reservoirs and ponds. The first set of methods are empirical models that predict trap efficiency, mostly of normally ponded large reservoirs using data on a mid to long-term basis. These models relate trap efficiency to a capacity/watershed ratio, a capacity/annual inflow ratio or a sedimentation index. Today, these models are the most widely used models to predict trap efficiency, even for reservoirs or ponds that have totally different characteristics from the reservoirs used in these models. For small ponds, these models seem to be less appropriate. They also cannot be used for predicting trap efficiency for a single event. To overcome these restrictions, different theoretical models have been developed based on sedimentation principles. These can be very simple, such as the overflow rate method, but also very complex when runoff and sediment are routed through a pond with incremental time-steps. The theoretical-based models are probably more capable of predicting trap efficiency for small ponds with varying geometric characteristics, and some of them also provide data on effluent sediment concentrations and quality. However, when reconstructing sediment yield values using sedimentation rates over a period of a few months to a few year (the mid-term basis), one needs a trap efficiency value for the whole period, not for one single event. At present only limited research has been done on establishing mid-term trap efficiency models based on theoretical principles. This is probably the most important gap in trap efficiency research.

Hydraulic behavior and removal efficiency of settling tanks

The performance of a rectangular settling tank with two effluent system is described by hydraulic laboratory experiments. The velocity profiles of neutral current by dye tracer method and LDA describe the effect of the two effluent system. The proper location of a baffle may be determined by using flow-through curves from dye tests. The experiments using the clay turbid water as settling suspension show that the overflow rate, the installation of baffle and the location of effluent are crucial influential factors on removal efficiency.

Settling velocity characterization of aquacultural solids

A top-loading settling column is described and used to characterize the settling properties of the solids in the discharge water from a commercial rainbow trout production facility. Mass-based and phosphorus-based settling curves are presented. The median settling velocity on a mass-basis for the settleable solids was 1.7 cm s −1. The median settling velocity for the settleable phosphorus was 1.15 cm s −1. Manually stripping fecal material from rainbow trout resulted in settleable solids with a median settling velocity of 0.7 cm s −1. Examination of the settling velocity curves show that halving the overflow rate (OFR) from 2 to 1 cm s −1 changes the removal efficiency from 0.61 to 0.73, an increase of about 20%. Halving the OFR again to 0.5 cm s −1 increases the removal efficiency to 0.81, an improvement of about 11%. Settling characteristics of aquacultural solids will vary from facility to facility. The methods described in this paper can be used to perform a similar type of analysis at other aquacultural sites, which may be growing other species under different management regimes.

Chernoff bounds for mean overflow rates

A number of independent traffic streams arrive at a queueing node which provides a finite buffer and a non-idling service at constant rate. Customers which arrive when the buffer is full are droppe...

A large deviation analysis of errors in measurement based admission control to buffered and bufferless resources

In measurement based admission control, measured traffic parameters are used to determine the maximum number of connections that can be admitted to a resource within a given quality constraint. The assumption that the measured parameters are the true ones can compromise admission controls measured parameters are random quantities, causing additional variability. This paper analyzes the impact of measurement error within the framework of Large Deviation theory. For a class of admission controls, large deviation principles are established for the number of admitted connections, and for the attained overflow rates. These are applied to admission to bufferless resources, and buffered resources in both the many sources and large buffer asymptotic. The sampling properties of effective bandwidths are presented, together with a discussion the impact of the temporal extent of individual samples on estimator variability. Sample correlations are shown to increase estimator variances procedures to make admission control robust with respect to these are described.

New Hypothesis for Secondary Clarifier Performance Under Hydraulically Limited Conditions

A new criterion called the effluent upflow velocity (EUV) is proposed as an important parameter affecting secondary clarifier performance under hydraulically limited conditions. Defined as the upflow velocity in the effluent withdrawal zone of a secondary clarifier, the EUV is proposed as having a direct link to clarifier performance in the presence of a wide range of particle size distributions in the effluent withdrawal zone. It is hypothesized that a wide range of particle size distributions in the effluent withdrawal zone of a clarifier could result from one of two factors: poor mixed liquor flocculation characteristics or strong density currents capable of resuspending previously settled floc. Consequently, in the presence of either of these factors, the EUV would likely show the best correlation with clarifier effluent suspended solids, more so than traditional hydraulic criteria such as the surface overflow rate or weir loading rate. Results from clarifier modifications completed at the County Sanitation Districts of Orange County's Wastewater Treatment Plant No. 2 in Huntington Beach, California, are presented in support of this hypothesis.

Classification evaluation of water injected hydrocyclone

The performance of hydrocyclone with a truncated cone and water injection facility at its apex has been studied. It was found that the flow rate of water in the overflow of the cyclone is higher when smaller cone is used. However on injecting water at the apex, the overflow water rate is minimum when smallest cone is used, implying localised effect of injection of water in the displacement of fines. Experiments using silica as feed, along with water injection, showed that the weight percent of particles less than a particular size in the underflow is reduced, this size being a function of inlet pressure and truncated cone diameter.

Continuous production of chitooligosaccharides using a dual reactor system

The continuous production of chitooligosaccharides from chitosan was achieved with a dual reactor system which consisted of an ultrafiltration (UF) membrane reactor and a column reactor packed with an immobilized enzyme. The production of the oligosaccharides was performed by two steps, the first, the preparation of the partially hydrolyzed chitosan (PHC) from viscose chitosan in the column reactor packed with an enzyme and the second, the production of the oligosaccharides from PHC in the UF membrane reactor. Three kinds of PHCs were obtained from three different outflow rates (3, 5, and 9 ml/min) in the column reactor and were supplied to a substrate feed tank of the following UF reactor in order to identify the influence of the feed on membrane fouling. The PHC obtained with a 5 ml/min overflow rate was the most suitable substrate for alleviation of membrane fouling and efficient hydrolysis under the operating conditions of the dual reactor system. In addition, chitooligosaccharides produced using the dual reactor system were readily fractionated by a proper selection of membranes. Therefore, it was shown that this dual reactor system was adequate for continuous production of chitooligosaccharides from chitosan by enzymic hydrolysis.

Evaluation of Correlations for Zone Settling Velocity Parameters Based on Sludge Volume Index‐Type Measures and Consequences in Settling Tank Design

Determining zone settling velocity (ZSV) parameters for use in flux‐theory analysis (design or operating charts) is time consuming. Correlations have been proposed for these parameters based on more readily obtained sludge volume index‐ (SVI‐) type measures. A number of these correlations are reviewed, and their ability to predict ZSV parameters is evaluated. This assessment uses a model to generate SVI values from specified ZSV parameters. The SVI values are applied in correlations to generate predicted ZSV parameters that are compared to the specified ZSV parameters. Specified and correlation‐generated ZSV parameters are used in the flux‐theory design procedure to assess the difference in maximum allowable overflow rate.

Reducing Memory Traffic and Accelerating Prolog Execution in a Superscalar Prolog System

Memory access operations constitute about 32.7% of all the operations executed in a typical Prolog program. Among these memory accesses, 75% are to the program control structures (environments and choice points). These memory accesses plus possible data cache misses greatly impair system performance, and the problem is even more severe in a VLIW, superscalar, or superpipelined Prolog system. This paper describes an innovative windowed register file management technique called SORWT (splittable overlapping register window technique). With SORWT, environments, choice points, and arguments can be stored in a windowed register file. SORWT reduces the number of memory accesses to only 25% of the number made when a conventional stack scheme is used. This paper describes in detail how Warren and PLM instructions can be implemented using SORWT, and it presents a register file overflow underflow handling mechanism called the memory window matrix (MWM) and a mapping function for use between register windows and the MWM. Thirty benchmark programs are used to study performance issues, the overhead of SORWT, optimal register file and window sizes, and the argument overflow rate.

Sludge Blanket Response to Storm Surge in an Activated-Sludge Clarifier

Aspects of operation and design for storm flows at a 190-m3/h (1.2-mgd) dry weather design flow activated-sludge plant were tested. A significant plant enlargement allowed recycle of test clarifier effluent to the main plant versus discharge to a stream. Thus it was possible to test high overflow rates without risk of a permit violation.On-line monitoring showed that the return sludge concentration increase lagged approximately 1 hour behind the start of the storm flow. This required an adjustment of the power equation model used. Furthermore, when the overflow rate exceeded the settling velocity of the feed solids, the sludge blanket rose faster than the model prediction. This phenomenon required developing a new model to predict blanket behavior under various loading conditions. The new model allows for plant design to more accurately include a clarifier zone for storing solids during peak flows associated with rain storms. An exponential equation, fitted to settling data, was also used in the new model with test data for comparison. The exponential equation gave a low blanket prediction. Results are compared with those of other research for solids storage during peak flow.

Factors influencing suspended solids concentrations in activated sludge settling tanks

A significant fraction of the total mass of sludge in an activated sludge process may be in the settling tanks if the sludge has a high sludge volume index (SVI) or when a hydraulic overload occurs during a rainstorm. Under those conditions, an accurate estimate of the amount of sludge in the settling tanks is needed in order to calculate the mean cell residence time or to determine the capacity of the settling tanks to store sludge. Determination of the amount of sludge in the settling tanks requires estimation of the average concentration of suspended solids in the layer of sludge ( X SB) in the bottom of the settling tanks. A widely used reference recommends averaging the concentrations of suspended solids in the mixed liquor ( X) and in the underflow ( X u) from the settling tanks ( X SB=0.5{ X+ X u}). This method does not take into consideration other pertinent information available to an operator. This is a report of a field study which had the objective of developing a more accurate method for estimation of the X SB in the bottom of the settling tanks. By correlation analysis, it was found that only 44% of the variation in the measured X SB is related to sum of X and X u. X SB is also influenced by the SVI, the zone settling velocity at X and the overflow and underflow rates of the settling tanks. The method of averaging X and X u tends to overestimate the X SB. A new empirical estimation technique for X SB was developed. The estimation technique uses dimensionless ratios; i.e., the ratio of X SB to X u, the ratio of the overflow rate to the sum of the underflow rate and the initial settling velocity of the mixed liquor and sludge compaction expressed as a ratio (dimensionless SVI). The empirical model is compared with the method of averaging X and X u for the entire range of sludge depths in the settling tanks and for SVI values between 100 and 300 ml/g. Since the empirical model uses dimensionless ratios, the regression parameters are also dimensionless and the model can be readily adopted for other activated sludge processes. A simplified version of the empirical model provides an estimation of X SB as a function of X, X u and SV f and can be used by an operator when flow conditions are normal.

Evaluation of Swirl-Flo™ Clarifier and UV/H2O2 Process for Domestic Wastewater Treatment

Pilot scale investigations were conducted to evaluate the efficacy of a novel process comprising of Swirl-Flo™ Clarifier (SFC) and Ultraviolet light/Hydrogen Peroxide (UV/H2O2) reactor for treatment of domestic wastewater. Overall Total Suspended Solids (TSS) and Chemical Oxygen Demand (COD) removal of over 82 % and 90 %, respectively was achieved in the plant. TSS removal in SFC was influenced by feed flow rate. TSS removal efficiency of about 45–50 % was achieved in SFC at flow rate of about 5.5–6.0 m3 hr−1 equivalent to overflow rate of 3.1–3.4 m3 m−2 hr−1. This compared favorably against an overflow rate of up to 1.0 m3 m−2 hr−1 obtainable in gravity settling units. COD removal in SFC was about 37–42 %, while that in UV Reactor was 18–23 % at H2O2 dosing of 800–900 ppm. Further polishing of the treated effluent by sand filter and activated carbon adsorption column yielded the effluent quality satisfactory for use as make up water in cooling towers for central air-conditioning plants. The treated effluent quality was consistent and showed little variation compared to the variation in influent wastewater characteristics.

Frequency domain heart rate variability and plasma norepinephrine level in the coronary sinus during handgrip exercise.

Heart rate (HR) variability has been recognized as an important noninvasive index of autonomic nervous activities. However, the relationship between HR variability and cardiac circulating norepinephrine (NE), especially with respect to coronary ischemia, remains unclear. This study was undertaken to determine whether HR variability indices can reflect cardiac NE levels during handgrip exercise. We simultaneously measured HR variability and cardiac NE overflow rate in 32 patients (30 men, 2 women) during a 6-min isometric handgrip exercise. Among the 32 subjects, 20 (19 men, 1 woman) had coronary artery disease (CAD) and 12 (control group; 11 men, 1 woman) did not. Hemodynamics and cardiac NE overflow rates among subjects at rest were not significantly different between the two groups. In the normal subjects, low-frequency (LF) spectra and LF/HF (high-frequency) ratios were not significantly changed during handgrip exercise, but HF spectra significantly increased from 10.1 +/- 4.5 to 12.2 +/- 7.0 ms (p < 0.05). In the subjects with CAD, LF and LF/HF spectra were significantly (p < 0.05 and 0.01, respectively) increased by handgrip exercise. High-frequency spectra were not significantly changed by handgrip exercise. In the normal subjects, a significant negative relation (r = -0.76, p < 0.01) was obtained between HF change and cardiac NE overflow rate, whereas this relationship was not significant in the subjects with CAD. The correlation between changes of LF/HF and cardiac NE overflow rate was significant in the normal (r = 0.56, p < 0.05) but not in subjects with CAD. These results suggest that vagal modulation of HR variability is more prominent in normal coronary artery subjects than in CAD subjects during handgrip exercise. Heart rate variability indices may thus serve as adequate indicators of autonomic nerve activity in subjects with normal coronary arteries but not in those with CAD, probably due to decreased adaptation to physical stress during handgrip exercise.

Effluent recycle and waste minimization in prawn farm effluent

Prawn farms withdraw huge quantities of sea water and discharge 5–30% of it back to the estuary during water exchange process. The effluent from the prawn ponds contains large quantities of solids, nitrogen, phosphorus and algae and it cannot be discharged into the estuary or ocean without proper effluent treatment. Sedimentation could form a partial treatment system. In this study, about 28–38% removal was achieved at an overflow rate of 40 m 3/m 2.d. Because of low loading, it requires a large area. Co-culture systems may be technically feasible but it is difficult to control the prawns growth. Considering the water quantity requirement, it is necessary to design a high rate treatment system to treat the effluent and recycle it in the system. A high rate floating medium filter led to 68% removal of suspended solids at 20 m 3/m 2.h and with a depth of 1.2 m. The removal efficiency increased when flocculant was added. A combined downflow floating medium and sand filter gave rise to almost 90% solids and phosphorus removal. Another way is to control the nutrient discharge by optimizing the feed and reducing its waste.

Evaluation of Swirl-Flo™ Clarifier and UV/H2O2 Process for Domestic Wastewater Treatment

Pilot scale investigations were conducted to evaluate the efficacy of a novel process comprising of Swirl-Flo™ Clarifier (SFC) and Ultraviolet light/Hydrogen Peroxide (UV/H2O2) reactor for treatment of domestic wastewater. Overall Total Suspended Solids (TSS) and Chemical Oxygen Demand (COD) removal of over 82 % and 90 %, respectively was achieved in the plant. TSS removal in SFC was influenced by feed flow rate. TSS removal efficiency of about 45–50 % was achieved in SFC at flow rate of about 5.5–6.0 m3 hr−1 equivalent to overflow rate of 3.1–3.4 m3 m−2 hr−1. This compared favorably against an overflow rate of up to 1.0 m3 m−2 hr−1 obtainable in gravity settling units. COD removal in SFC was about 37–42 %, while that in UV Reactor was 18–23 % at H2O2 dosing of 800–900 ppm. Further polishing of the treated effluent by sand filter and activated carbon adsorption column yielded the effluent quality satisfactory for use as make up water in cooling towers for central air-conditioning plants. The treated effluent quality was consistent and showed little variation compared to the variation in influent wastewater characteristics.

Modeling and control of a continuous crystallization process Part 2. Model predictive control

Multi-input–single-output (MISO) (the output variables are related to the crystal size distribution (CSD), crystal purity, and production rate, and the input variables are the fines dissolution rate, clear liquor or overflow rate, and the crystalllizer temperature) and multi-input–two-output (MITO) model predictive control of the KCl cooling crystallizer described in Part-1 of this two-part paper is investigated. The process model is the same linear ARX or the non-linear neural network models developed in Part-1. The optimization is performed by the feasible sequential quadratic programming (FSQP) algorithm (Zhou and Tits, 1992). It is shown that the non-linear MPC provides a satisfactory controller for the multivariable control of the crystallization process.

FIELD MEASUREMENT OF THE SATURATED HYDRAULIC CONDUCTIVITY OF A MACROPOROUS SOIL WITH UNSTABLE SUBSOIL STRUCTURE.

A field method for measuring saturated hydraulic conductivity, K s , was developed to characterize water flow in highly-weathered soils of Sitiung, Indonesia. Soils in this area are known to absorb large volumes of rainwater rapidly. However, K s data obtained on soil cores do not corroborate field-observed rapid infiltration rates. In the field method, a constant rate irrigation was applied to a field plot, delineated to a depth of 120 cm, and bordered on the surface to contain a depth of ponded water. The rate of irrigation was sufficient to maintain the ponding depth at a constant level as well as cause water to overflow from the ponded surface. The difference between the steady-state irrigation and overflow rates was considered to be the instantaneous flux and was assumed applicable to all depths. Simultaneous tensiometric measurements of pressure head as a function of depth provided the hydraulic gradients needed for calculation of K s using Darcy's law. Hydraulic gradients deviated considerably from unity, and soil saturation did not exceed 92% of porosity. Laboratory-measured K s values for the stable-structured topsoil agreed well with the field data. However, those for the subsoil were 2 to 3 orders of magnitude lower than the field-measured values. The susceptibility of the subsoil to compaction during core extraction and slaking when in contact with free water appeared to be responsible for the highly reduced rates of flow in the laboratory samples. The subsoil pore structure was preserved only as long as it was overlain by the stable structured topsoil. Results suggest that measurements of water flow on small soil cores may, in some cases, be of questionable value. The field method provided accurate in situ data on plot-size areas. The field plot method used in this study causes minimal disturbance of the soil while the effects of sample confinement and overburden are represented fully in the measurements.

Winter temperature gradients in circular clarifiers

To understand the effect of temperature‐induced density currents on circular clarifiers, a field study was performed on three uncovered circular tanks in Oregon during periods of low flow and high surface cooling. Temperatures were measured as a function of radial position and tank depth. Meteorological parameters were also measured to compute the surface heat flux from the water surface. Water temperatures in the tank were often 20 to 30 °C warmer than the equilibrium temperature. These studies showed that density currents and unstable temperature gradients were common. Surface temperatures were often approximately 1 °C cooler than bottom temperatures. The momentum and suspended solids of the inflow caused a density current that moved along the clarifier bottom and then rose near the middle of the tank. In the winter, this density current rose as a result of the decrease of initial momentum of the inflow, the buoyancy of the inflow after suspended solids had settled, and the geometry of the tank. Also, vertical velocities induced by surface cooling were two orders of magnitude greater than the overflow rate, suggesting that particles could be kept in suspension by convective currents.

An empirical model for primary sedimentation of sewage

Theoretical mathematical models of primary sedimentation and column settling tests have failed until now to predict the behaviour of sedimentation tanks under actual operating conditions due to the difficulties in simulating the effect of the density currents and the complex phenomenon of flocculation. Therefore, empirical models can be helpful in the design of sedimentation tanks. Using performance data from three different pilot-scale sedimentation tanks, empirical mathematical models were developed in this paper relating suspended solids (SS) removal efficiency to surface overflow rate, influent SS concentration, and sewage temperature. The model coefficients were derived from the combined analysis of three well correlated sets of data, thus giving a good indication for their possible general applicability. The analysis of experimental data also gave a relationship between SS and chemical oxygen demand (COD) removal efficiencies.