Journal of Environmental Engineering

The solids flux theory has traditionally been used for clarifier design. However, it's use for analysis of clarifier behavior in operating plants has been largely overlooked due to the intensive data requirements and complex graphic analysis. It was previously demonstrated that the theory can be presented as simple operating curves which can be used to predetermine blanket failure. Elsewhere, new control boundaries were used in the derivation of the theory, making possible the prediction of the rate of rise or fall of blankets in laboratory‐scale clarifiers. In the present paper, the newly configured model is used to predict blanket performance in full‐scale clarifiers. The predicted rates correlate well with the observed rates $(r^2=0.93)\text{.}$ The effects of measurement error, such as flow rates and settling rates, on the ability to predict the performance of the sludge blanket are studied. Errors associated with predicting the blanket performance outweigh errors in observing the movement of the blanket and are in agreement with the magnitude of scatter in the actual data.

An assessment has been made of fluid turbulence intensities and local velocities throughout a 1‐L square reactor in relation to flocculation efficiency. Measurements have been taken using laser‐Doppler anemometry for three types of stirrer, and aluminum sulphate has been used as the coagulant for a synthetic raw water based on kaolin as suspension with controlled alkalinity. Stirrer speeds ranging from 16 to 100 rpm were adopted, and the torque applied to the fluid was measured directly. Turbulence intensity is shown to be an alternative measure of flocculation efficiency to velocity gradient or power input. The power input required for minimum turbidities is much the same for all stirrers, but where optimum conditions cannot be predetermined, a stirrer that extends throughout the volume of the reactor and has sharp‐edged blades is shown to be more versatile than the other types.

The sulfuric acid that causes corrosion of sewer crowns is generated by a complex microbial ecosystem. Several species capable of oxidizing sulfur compounds colonize exposed concrete. A microbial succession occurs that is ultimately dominated by Thiobacillus thiooxidans. Heterophilic symbionts consume inhibitory waste products. Characteristics of this system were investigated. Surface pH and corrosion rates of new concrete in an experimental chamber exposed to sewer gases were monitored. After an initial period in which pH varied sharply over periods of hours, days, or weeks, corrosion‐inducing microorganisms were well established. The corroding surfaces were flushed intermittently with sewage to determine whether this might serve as a control method. Heavy and frequent flushing will be necessary to significantly reduce corrosion rates.

From October 1986 to December 1987, a study on the evolution ot the primary productivity in a wastewater stabilization pond was carried out in situ using the C‐14 technique. Most primary productivity measurements are limited to relatively short exposure times $(\simeq 2\mathrm{hr})\text{,}$ this creates problems in estimating daily rates, although these difficulties could be overcome by summing experiments performed over a whole day. However, for routine purposes such procedure is impracticable. The only way to reach meaningful estimates is the use of theoretical models. A semiempirical model is applied for the evaluation of primary productivity in a sewage pond. The data base necessary for the model development are key nutrients, light, and temperature. The inorganic phosphorus and nitrogen and the inorganic carbon (as free ${\mathrm{CO}}_2$ present in the local environment of algal cells) are the limiting factors for the phytoplanktonic development. On the other hand, the photosynthetic activity depends mainly on the light intensity and temperature. The Nelder and Mead method was used for the calibration of the model.

Experiments were carried out in a reduced‐scale pilot‐plant model of a grit chamber, having a cross‐sectional geometry similar to that of a conventionally aerated grit chamber. The model using clear water was built on a geometric scale of 1:4. Nozzles along one wall of the chamber injected jets of water, which caused a helical rotating motion of the water in the chamber. Three sizes of nozzles were tested, as well as two nozzle spacings. Water velocities at various points were measured under different operational conditions. A consistent correlation between power input and transverse bottom velocity was derived for the model grit chamber. This correlation enables the calculation of power input, which is required for a desired bottom velocity. It was concluded from the operation of the model that water jets can replace air diffusers used in conventionally aerated grit chambers. This system may reduce odor nuisance, which is caused by aeration in the conventional design.

A simple modification of the conventional countercurrent air‐stripping process, introduced as cascade air stripping, is proposed for cost‐effective removal of semi‐ and low‐volatile organic contaminants from water. This paper summarizes the results of a pilot‐scale demonstration study and a prototype‐scale feasibility study undertaken to demonstrate the concept of cascade air‐stripping system, and to evaluate its performance against that of the conventional air‐stripping (AS) system. Pressure drops and energy requirements of the two systems were compared under a range of water and air loading conditions in the pilot‐ and prototype‐scale versions. Removal capabilities of the two systems were compared over a wide range of volatilities. When compared on the basis of air‐side energy input, the proposed cascade AS system offers some advantages over the conventional AS process: accommodating 35–50% more air flow; achieving comparable removals at 40–50% of the packing depth; and attaining considerably lower effluent concentrations.

Extensive field measurements aimed at studying short‐term dissolved oxygen (DO) variability have been carried out in a marine fish culture zone in Hong Kong. A dynamic oxygen budget model is developed to analyze the diurnal water‐quality data from a series of 26‐hr surveys. The model formulation includes phytoplankton dynamics and photosynthetic production, sediment oxygen demand (SOD), fish respiration, carbonaceous and nitrogenous oxidation, and atmospheric oxygen transfer. The model is used to determine the components of oxygen consumption and production—in particular, the SOD and carbon‐to‐chlorophyll (CCHL) ratio. It is found that SOD and algal respiration are major oxygen consumption items, and the CCHL ratio is much higher than reported in previous mathematical model studies. Simulations of the diurnal DO variation are compared with data over a wide range of meteorological and ecological conditions. The model interpretation provides a basis for the formulation of a water‐quality model for short‐term DO forecasts

Short‐term trends in dissolved oxygen in a weakly flushed marine fish culture zone in Hong Kong are studied by a real‐time water‐quality model with seven system variables: dissolved oxygen, chlorophyll‐a, organic nitrogen, ammonia, nitrate, zooplankton, and sediment algal carbon. The carbon‐chlorophyll ratio is computed as a function of a time‐variable saturating light intensity. Sediment oxygen demand is related to organic loads from the fish farm as well as algal biomass. Water‐quality predictions are compared with well‐documented field observations for both background and algal bloom conditions, over time scales on the order of a month. Model simulations demonstrate the importance of light intensity and photosynthetic production in dissolved oxygen variations, which are well predicted. The model also provides useful insights into the causes of severe oxygen depletion and phytoplankton dynamics in a eutrophic subtropical coastal marine ecosystem.

Sludge disposal is a nuisance for municipal wastewater treatment plants. Increasingly stringent environmental control regulations have resulted in limitations on sludge disposal options, and hence there has been a resulting increase of interest in disposal by incineration. In this study the potential of using dewatered sludge mixed with clay as lightweight coarse aggregate after incineration was examined, and the properties of the aggregate as well as the resulting concrete were investigated. From the tests conducted on 10% fines, bulk density, specific gravity, porosity, water absorption, and compressive strength, it was found that clay‐blended sludge could be used to produce lightweight aggregates.

Existing land treatment systems in the food-processing and pulp-and-paper industries are surveyed to obtain site and operation information. The results indicate that: (1) Land treatment is used as a waste-management technology at 117 facilities in these industries; (2) application of solids is common in the pulp-and-paper industry while slow-rate irrigation of wastewater is more common in the food-processing industry; and (3) grasses are the most common crop although multicropping of corn, alfalfa, grains, and grasses occurred. In these industries, more than 60,500,000 m³ (16,000,000 gal.) of wastewater and about 300,000 dry metric tons (328,000 dry tons) of solid waste are applied annually. Most sites are less than 80 ha (200 acres) although one was 970 ha (2,400 acres).

A dynamic computer model of a pump station at a wastewater-treatment plant was developed. The model predicts pumping rates and wet well levels as a function of time. The pumps included both constant- and variable-speed pumps. Input included influent flows, pump head versus discharge curves, pump cut-in and cut-out elevations, and changes in speed for variable speed pumps. The model accurately predicted water levels in the wet well after calibration and identified problems in the original pumps sequencing.

The computation of the effectiveness factor for spherical shells, employing Michaelis-Menten kinetics or its more general form, which incorporates the effect of substrate inhibition, involves solving a steady-state continuity differential equation. This study questions the accuracy of Stevens et al.'s solutions of 1987 and 1988 obtained by the collocation method, and advocates the easy-to-use and more effective shooting method for this type of biofilm kinetic analysis.