Quality Model

The purpose of the quality model is to simulate pollutant loading and buildup, washoff, as well as individual domestic, commercial and industrial contributions, and transport. Seven different types of quality analyses can be carried out by InfoSewer/Pro and are explained below.

POLLUTANT TRANSPORT

InfoSewer/Pro can effectively simulate the transport of dissolved pollutants throughout the sewer collection system. It tracks the movement of conservative constituents (e.g., chloride, bromide, sulfate, boron, sorbed trace metals) flowing through the network over time. The dynamic quality simulation model is predicated on conservation of mass coupled with reaction kinetics and consists essentially of three processes: advection in pipes, mixing at sewer manholes and wet-wells, and kinetic reaction mechanism. Longitudinal dispersion due to concentration gradient is neglected which means that there is no intermixing of mass between adjacent sewage parcels traveling down a pipe. Advective transport within a sewer pipe is represented with the following equation:

....…………………………… (25)

where Ci is the concentration (mass/volume) in pipe i as a function of distance x and time t; ui is the flow velocity (length/time) in pipe I; and r is the rate of reaction (mass/volume/time) as a function of concentration.

For conservative (inert) pollutants the rate of reaction (r) is set to zero. For pumps, instantaneous substance advection is assumed.

At sewer manholes and wet-wells, the mixing of fluid is taken to be complete and instantaneous. Thus the concentration of a substance in sewage leaving the manhole or wet-well is simply the flow-weighted sum of the concentrations from the incoming pipe(s) and is described by the following equation:

....…………………………… (26)

where Qin is the incoming flow (volume/time); Cin is the pollutant concentration of the incoming flow; and Cout is the concentration of the pollutant leaving the manhole.

Under completely mixed conditions, the concentration throughout the wet-well is a blend of the current contents and that of any entering sewage and is represented by the following equation:

....…………………………… (27)

where Vww is the wet-well volume at time t, Cww is the wet-well concentration; and the remaining terms are as defined above.

The ability to model pollutant transport in sewer collection systems is useful in determining the amount of pollutants that is transported to the wastewater treatment plant and assessing impact on the receiving waters.

TIME OF CONCENTRATION

InfoSewer/Pro can model the changes in the age of sewage flow (time of concentration) throughout a collection system. Time of concentration is the time spent by a sewage flow parcel in the network (i.e., the time of flow in the sewerage system). This parameter is useful to address important water quality and safety issues such as generation of sulfide that may occur in a sanitary sewer system (which manifest itself in corrosion and odor issues).

In InfoSewer/Pro, new sewage entering the network from loading manholes enters with age of zero. As this sewage moves through the collection system it splits apart and blends together with sewage flow parcels of varying age at manholes and wet-wells. InfoSewer/Pro provides automatic modeling of sewage age. Internally, it treats age as a reactive constituent whose growth follows zero-order kinetics with a rate constant (r =1.0) equal to 1 (i.e., each second the sewage becomes a second older). Time of concentration for a manhole is thus calculated as a flow-weighted average sewage age value of flows entering the manhole. Travel time of a sewage flow parcel through a sewer pipe is computed based on flow velocity and pipe length. For the pipes leaving a manhole, the average sewage age is increased by the travel time to the next downstream manhole.

SOURCE TRACING

InfoSewer/Pro can also perform sophisticated source tracing calculations. Source tracing tracks over time what percent of sewage reaching any pipe or manhole in the network had its origin at a particular source node. The source node can be any manhole in the network, including wet-wells. Source tracing is very useful in sewer collection systems, and could be used for (1) tracking changes in sewage flow contribution (and associated constituents) over space and time; (2) predicting impact of industrial and commercial waste discharges on performance of wastewater treatment plants; (3) determining contaminant level that causes a wastewater treatment plant to be in violation of its discharge permits; (4) and developing appropriate user charges based on wasteloads and level of contaminant. Internally, InfoSewer/Pro treats the source node as a constant source of a non-reacting constituent that enters the network with a concentration of 100.

BIOCHEMICAL OXYGEN DEMAND

Biochemical Oxygen Demand (BOD) is the most widely used parameter of organic pollution in sanitary sewer systems. The ability to model BOD is of great importance in wastewater engineering to assist wastewater utilities in (1) estimating the quantity of oxygen required to biologically stabilize the organic matter present; (2) determining the size of wastewater treatment facilities; (3) evaluating the efficiency of the treatment process; and (4) ensuring compliance with wastewater discharge permits (Tchobanoglous 2003).

InfoSewer/Pro models the rate of BOD oxidation (exertion) throughout the collection system using first-order kinetics with the rate of oxygen utilization being proportional to the difference between the amount of oxygen used and the ultimate BOD as:

………………………………(28)

or

………………………………(29)

where

BOD = BOD exerted at time t, mg/L

UBOD = total or ultimate carbonaceous BOD, mg/L

k = first-order reaction rate constant, 1/day

t = time, day

The value of the first-order reaction rate constant k varies with the type of waste and normally ranges from 0.05/day to 0.46/day or more, with a typical value of 0.23/day. This constant can also be expressed as a function of temperature as:

………………………………(30)

where k20 is the first-order reaction rate constant at 20oC (1/day) and T is the temperature (degree Celsius). The value of θ is 1.056 in the temperature range between 20oC and 30oC and 1.135 in the temperature range between 4oC and 20oC, with a typical value of 1.047. InfoSewer/Pro determines and uses appropriate values of k and θ based on the user-specified temperature. The default values used by InfoSewer/Pro for k, θ, and T are 0.23/day, 1.047, and 200C, respectively.

SEDIMENT TRANSPORT

Sanitary sewer systems can carry substantial loads of suspended solids (waste solids). These sediments are complex mixtures of cohesive (organic) and non-cohesive (minerals) materials and exhibit a wide range of particle sizes and densities. They can collect causing blockages (shock loading under periods of low flow) and overflow events, as well as impairing the hydraulic capacity of the sewer pipes (by restricting their flow area and increasing the bed friction resistance). In addition, when waste solids are intermittently agitated and moved along the pipes, sulfide generation is increased which may cause various problems including odor, hazard to maintenance crews, and corrosion of unprotected sewer pipes produced from cementitious materials and metals. Limiting velocity criteria are generally adopted when designing sanitary sewer systems to control sediment deposition in the pipes.

Wastewater flow velocity and sediment transport in sewer systems are interdependent. As the flow increases from zero, flow-induced forces (lift and drag) acting on the sediment particle increase. When these forces exceed the submerged weight of the particle, the sediments start to move. With a further increase in velocity, the particles will be suspended by eddies of fluid turbulence and move downstream with the wastewater. With a subsequent decrease in flow velocity (beyond the limiting or terminal velocity), sediment particles will start to settle by gravity at a rate proportional to their settling velocity. A further increase in flow velocity provides the energy to scour and transport the deposited material along the sewer pipes. This dynamic process continues based on the flow conditions inherent in the sewer system.

InfoSewer/Pro can simulate the transport and gravitational settling of sediments (total suspended solids including grit) over time throughout the sewer collection system under varying hydraulic conditions. As long as flow velocity exceeds the critical/terminal velocity, InfoSewer/Pro assumes that the sewage flow has the capacity to transport all incoming sediments. Deposited sediment particles are also assumed to be scoured and transported downstream when velocity of the sewage flow exceeds the terminal velocity. Settling starts when flow velocity falls below the critical velocity. In the model, transport of the sediment particles is governed by advection (Eq. 24) implying that the particles are transported at local flow velocity. The model assumes that the sewer pipe diameter and roughness coefficient remain constant and are unaffected by sediment deposition.

HYDROGEN SULFIDE

The control of odorous gases and the corrosion of sewers are the two most important problems in operating wastewater collection systems. Evaluation of existing or potential odor or corrosion problems, and identification of where such problems will occur is, therefore, highly essential. In sanitary sewer systems, odors are produced as a result of biological decomposition of organic matter, particularly those containing sulfur and nitrogen, under anaerobic conditions prevailing in the slime layer of gravity pipes, force mains, and wet wells. Hydrogen sulfide (H2S) and ammonia are the only malodorous inorganic gases produced from the decomposition. Other odor producing substances include organic vapors such as idoles, skatoles, mercaptans and nitrogen-bearing organics. However, H2S is the most commonly known and prevalent odorous gas associated with domestic wastewater collection and treatment systems. H2S DetectorTM extension for InfoSewer/Pro suite gives wastewater engineers a powerful Operations and Maintenance (O&M) tool to readily model and analyze entire sewer collection systems for sulfide generation and corrosion potential under varying conditions anticipated throughout the life of their systems.

Hydrogen sulfide has a characteristic rotten egg odor, is extremely toxic, is corrosive to metals, and is a precursor to the formation of sulfuric acid (which corrodes concrete, lead-based paints, metals, and other materials). The conditions leading to formation of H2S generally favor the production of other odorous organic compounds. Therefore, investigation of the conditions favoring H2S formation not only helps to quantify the potential for odor generation from other compounds, but also it aids in identifying potential corrosion problems in the collection system.

The occurrence of H2S in wastewater collection systems, other than that added from industrial sources and infiltrated groundwater, is primarily the result of the reduction of sulfate ion ( ), one of the most universal anions occurring in natural waters, under anaerobic conditions, as shown by the following reaction.

The molecular H2S, formed from sulfate reduction, dissolves in the waste water and dissociates in accordance with reversible ionization reactions, expressed as:

The partitioning of the hydrogen sulfide into these components (i.e., (H2S) aqueous, HS- ion, and S= ion) depends primarily on the temperature and the pH of the wastewater, although ionic strength, as represented by dissolved solids or electrical conductivity, also affects the partitioning. The HS- ion and S= ion produce no odors. Some of the aqueous H2S will escape into the sewer atmosphere causing the odor problem. The concentration of H2S gas in the atmosphere will vary with the concentration of (H2S) aqueous according to Henry’s law. The rate of escape of H2S gas is a function of the difference between the saturation or equilibrium concentration determined by Henry’s law and the actual concentration of H2S in the sewer atmosphere. The EPA provides the figure below that shows H2S in the sewer atmosphere in equilibrium with the given concentrations of aqueous H2S concentration in the wastewater at the respective temperatures, for a pressure of one atmosphere.

Prediction of the rate of sulfide buildup and corrosion potential is an essential element in the design of new sewer systems as well as in the evaluation of existing systems. The rate of sulfide buildup depends on a number of environmental conditions, including, concentration of organic material and nutrients, sulfate concentration, dissolved oxygen (DO), pH, temperature, stream velocity, surface area, and detention (residence) time. Accounting for all these environmental conditions, H2S Detector predicts sulfide buildup in sewer collection systems for gravity sewers, force mains, and wet wells using the Pomeroy-Parkhurst equations. H2S Detector enables wastewater utilities to pinpoint odor and corrosion problems, develop effective monitoring programs, alert plant operators and sewer maintenance workers to potential danger and the need to observe safety practices, and evaluate and implement effective control system such as aeration, chlorination, and mechanical cleaning. As described above, H2S is an acutely toxic material. It can cause serious health hazards even at very low concentrations. The physiological effects (i.e., toxicity spectrum) of H2S are summarized in the following figure provided by EPA.

CORROSION PREDICTION

Corrosion is one of the primary reasons that sewer systems lose their structural integrity. Corroded sewer pipes may allow greater inflow and infiltration into the collection system, further deteriorating reliability of the network by causing undesirable conditions such as surcharges and overflows, ultimately requiring premature replacement of the pipes. Corrosion of unprotected concrete or metal surfaces is primarily due to the production of sulfuric acid in sewer systems through oxidation of hydrogen sulfide gas by bacterial action on the exposed surfaces under aerobic conditions. This type of corrosion is commonly referred to as microbially induced corrosion. Corrosion PredictorTM extension for InfoSewer/Pro suite helps wastewater engineers to predict the rates of corrosion of the sewer pipes in their collection systems under varying environmental and hydraulic conditions. It enables them to pinpoint corrosion problems, prioritize repairs, specify corrosion resistant materials or select other forms of corrosion protection (e.g., protective linings).

As previously described in the hydrogen sulfide section, some of the soluble H2S available in pipes may escape into the sewer atmosphere and transferred to the pipe walls above the wastewater surface. It will then be taken up as it comes in contact with the damp surfaces of pipes. The H2S retained in this dampness is then converted to sulfuric acid by aerobic bacteria, as described in the following reaction:

Next, some or all of the produced sulfuric acid reacts with the pipe material causing corrosion, mainly at the inside pipe wall above the wastewater flow line. The amount of sulfuric acid that reacts with the pipe material depends on the rate of production of the acid, which in turn depends on moisture, the presence of oxygen, and the mass emission of sulfide gas. If the rate of acid production is slow, almost all of the acid will react with the pipe material. If the rate of production is rapid, much of the acid will not be able to diffuse through the material. Consequently, it will be carried down the walls of the pipe and into the flowing wastewater stream where the sulfuric acid reacts with alkalinity producing sulfate ion.

In addition to the concentration of acid present, the corrosive effect of sulfuric acid varies according to the type of pipe material used and the ambient temperature. Cementitious pipes, including ferrous pipes with mortar lining, experience a reaction that converts the surface material into a pasty mass, which is primarily a calcium sulfate (CaSO4), commonly referred to as gypsum. This pasty mass may fall away and expose new surfaces to corrosive attack. Ferrous pipe materials may experience surface reaction in which a portion of the material is dissolved and a portion is converted to iron sulfide, yielding a hard bulky mass that forms on the exposed surface. A warm and humid environment creates good condition for microbial induced corrosion.

The rate of pipe corrosion depends upon the rate of sulfuric acid production, the amount of the produced sulfuric acid that reacts with the pipe material, and the alkalinity of the pipe material. Sulfuric acid production is related to the amount of hydrogen sulfide gas that escapes to the sewer atmosphere, which in turn depends on a number of environmental and hydraulic conditions previously described in relation to hydrogen sulfide buildup, partitioning, and release. Corrosion PredictorTM uses the following equations to estimate rate of corrosion for cementitious (cement-bonded) materials and ferrous materials in gravity sewers. In force mains, where the lines flow full, there is generally no internal corrosion since generation of sulfuric acid is prevented.

For cement-bonded materials (Metcalf & Eddy 1981),

………………………..(31)

where

C = average rate of penetration, mm/yr

k = coefficient of efficiency for acid reaction considering the estimated fraction of acid remaining on the wall May be as low as 0.3 and approaches 1.0 for complete acid reaction

= flux of H2S to the pipe wall, g/m2.hr.

A = alkalinity of the cement-bonded material, expressed as CaCO3 equivalents. Approximately 0.18 to 0.23 for granitic aggregate concrete, 0.9 for calcareous aggregate, 0.4 for mortar linings, and 0.5 for asbestos cement.

For ferrous materials (Metcalf & Eddy 1981),

……………………………..(32)

where C, k, and are consistent with the definitions given above. The default values used by Corrosion PredictorTM for k and A are 0.5 and 0.5, respectively.