Seawater Infiltration & Sulfate

Introduction

Many wastewater plants suffer capacity loss due to inflow and infiltration. For the majority of municipal treatment plants there is likely to be a seasonal impact resulting from rain events, snowmelt, spring thaw, etc. But there are also wastewater plants located next to or near the ocean that suffer from infiltration every day due to seawater intrusion. The impact of high influent sulfate concentrations from seawater intrusion on anaerobic digestion will be the focus of this post. Specifics of the particular plant are shown in the simple block diagram below.

Definitions

In any discussion of inflow and infiltration I think it is best to go to one of the best sources of information for our definition of these terms. The source I’m going to quote from is the Office of Water Programs, California State University, Sacramento textbook “Operation and Maintenance of Wastewater Collection System,” Volume 1, Seventh Edition, 2015.

Inflow

Water discharged into a sewer system and service connections from such sources as, but not limited to, roof leaders, cellars, yard and area drains, foundation drains, cooling water discharges, drains from springs and swampy areas, around manhole covers or through holes in the covers, cross-connections from storm and combined sewer systems, catch basins, stormwaters, surface runoff, street wash waters, or drainage. Inflow differs from infiltration in that it is a direct discharge into the sewer rather than a leak in the sewer itself.

Infiltration

The seepage of groundwater into a sewer system, including service connections. Seepage frequently occurs through defective or cracked pipes, pipe joints and connections, interceptor access risers and covers, or manhole walls. Similar to, but the opposite of, EXFILTRATION.

Exfiltration

Water wastes and water-carried wastes that unintentionally leak out of defects in a sewer pipe wall and into the environment.

Seawater Intrusion

Composition of Seawater

In the table below you can see the "typical" concentration of various ions in seawater. To be noted in particular for this discussion is the concentration of sulfate which has an average "expected" concentration of 2,680 mg/L.

In contrast to seawater, untreated domestic wastewater typically has a sulfate concentration between 20 and 50 mg/L with an expected average value of 30 mg/L for medium-strength wastewater. Values outside this range would usually indicate that oxidized sulfur compounds such as sulfate, sulfite, and thiosulfate may be present in significant concentrations due to a contribution from various industrial wastewaters. These compounds can serve as electron acceptors for sulfate-reducing bacteria, which consume organic compounds in anaerobic digesters and produce hydrogen sulfide which I'll discuss in more detail in just a moment.

As can be seen in the graph below, the actual influent sulfate values at this municipal wastewater plant are 7 to 20 times greater than what would normally be expected. To produce such large values requires a contributing source that either generates a high volume of flow and/or a high sulfate concentration. After a great deal of research and analysis the source of the high influent sulfate was determined to be tidal inflow. The tidal inflow to this coastal wastewater treatment plant provides both a large volume of flow and a high sulfate concentration. Based on wet weather flow monitoring conducted at 31 locations, it was determined that, on average, 0.38 million gallons per day (MGD) of tidal inflow enter the sanitary sewer system. The peak tidal inflow during the sampling period was estimated to be 2,552 gpm (peak hourly average) and 0.76 MGD (daily total).

From the summary statistics tabulated below you can see the mean influent sulfate concentration for this coastal wastewater plant was 408 mg/L. This represents a 1,260% increase over the typical influent sulfate concentration of 30 mg/L. The lesson here is simple. If you are a coastal wastewater plant and wonder if you are suffering from seawater intrusion, start measuring your influent sulfate concentration. Do so at random times but correlate your sampling with high and low tide conditions to determine if the seawater intrusion is continuous or if it spikes during high tide.

Impact of Influent Sulfate on Anaerobic Digestion

This wastewater plant is a conventional activated sludge system with anaerobic digesters. In the first picture the failed floating digester covers are being removed. This was just one capital improvement project among many other upgrades that had been done to improve performance. In the second photo the new digester covers are in place.

The focus on influent sulfate was the result of concerns about the hydrogen sulfide (H2S) content in the anaerobic digester gas. Previous digester gas testing had shown extraordinarily high H2S values in the range of 7,000 to 17,000 mg/L, which, quite frankly, was hard to believe until the high influent sulfate concentrations were found. Free hydrogen sulfide concentrations of 50 mg/L can inhibit the activity of methanogenic bacteria by about 50% resulting in a significant reduction in methane production. Complete inhibition of methanogens can occur at free sulfide concentrations of approximately 250 mg/L. The mechanisms by which sulfates are converted to hydrogen sulfide are shown in the graphic below.

Chemical Binding of Sulfide

Hydrogen Sulfide Review

Wastewater influent is commonly tested for sulfate (SO4-2) to asses its potential for odor formation and the treatability of sludge. Hydrogen sulfide results from the reduction of sulfate to hydrogen sulfide gas by bacteria under anaerobic conditions as follows:

The concentration of oxidized sulfur compounds in the influent to an anaerobic digester is important, as high concentrations can have a negative effect on anaerobic treatment. Sulfate-reducing bacteria compete with the methanogenic bacteria for COD and thus can decrease the amount of methane gas production so critical to a cogeneration project. While low concentrations of sulfide (less than 20 mg/L) are needed for optimal methanogenic activity, higher concentrations can be toxic as previously stated.

Because un-ionized H2S is considered more toxic than ionized sulfide, pH is important in determining H2S toxicity. The degree of H2S toxicity is also complicated by the type of anaerobic biomass present (granular versus dispersed), the particular methanogenic population, and the feed COD/SO4-2 ratio. With higher COD concentrations, more methane gas is produced to dilute the H2S and transfer more H2S to the gas phase.

Note: Ionized refers to the state in which an atom is missing one or more of its electrons and is, therefore, positively charged. An ionized gas is one in which some or all of the atoms are ionized as compared to being electrically neutral. The ionized electrons behave as free particles in the gas.

Hydrogen sulfide exists in aqueous solution as either the hydrogen sulfide gas (H2S), the ion (HS‾), or the sulfide ion (S2-), depending on the pH of the solution. At a pH of approximately 9, more than 99% of the sulfide dissolved in wastewater occurs in the form of the non-odorous hydrosulfide ion (HS‾). Therefore, odorous amounts of hydrogen sulfide gas will not be released if a pH above 8 is maintained. Below this pH value, hydrogen sulfide gas is released from the wastewater. In anaerobic digesters the optimal pH is between 6.5 and 7.5, a range that allows the release of H2S of up to almost 70 percent as shown in the graph below.

Note: An aqueous solution is a solution in which the solvent is water rather than, for example, alcohol or ether. It is usually shown in chemical equations as a subscript (aq).

Anaerobic Digester Gas Quality

The quality and quantity of digester gas produced can also be used to evaluate digester performance. Gas production is directly related biochemically to the amount of VS destroyed and is expressed as volume of gas per unit of mass of VS destroyed. The specific gas production rate is different for each organic substance in the digester. The table below gives the specific gas production for several common organic substances found in wastewater.

The range of gas production varies from approximately 1.2 to 1.5 m3/kg (20 to 25 ft3/lb) of VS destroyed for fats to 0.7 m3/kg (12 ft3/lb) of VS destroyed for proteins and carbohydrates. A typical municipal anaerobic digester handling primary sludge and waste activated sludge, as in Richmond, should produce approximately 0.8 to 1.0 m3/kg (13 to 18 ft3/lb) of VS destroyed. The amount of gas produced is a function of temperature, SRT, and VS loading. Specific gas production should be measured until an average value can be obtained and used for monitoring.

In addition to trace amounts of nitrogen, hydrogen, and hydrogen sulfide, methane and carbon dioxide are the two main constituents of digester gas. Expected performance data from healthy digesters suggest methane concentrations of 60 to 70%, by volume, and carbon dioxide concentrations of 30 to 35%, by volume. A digester gas analysis from a sample taken on 8/11/2006 showed the following composition: methane = 59%, carbon dioxide = 37%, and nitrogen = 4%. An analysis of anaerobic digester gas constituents from several different wastewater treatment plants is shown in the table below.

Sulfides may be introduced into the anaerobic digester as a component of the wastewater or may be produced by the biological reduction of sulfates or degradation of proteins which contain sulfur. Some of the sulfide leaves the system as hydrogen sulfide gas, while a portion of the sulfide will be precipitated as heavy metal salts if these metals are present. However, a portion of the sulfide remains dissolved as a weak acid that ionizes depending on the pH of the solution. The presence of hydrogen sulfide can indicate unbalanced digestion, industrial waste sources, or, as in the case of Richmond, saltwater infiltration. As previously discussed, sulfides can be toxic to bacteria in an anaerobic system at concentrations in excess of 200 mg/L at a pH near neutral. However, at concentrations between 50 and 100 mg/L, sulfides are tolerated with little or no acclimation.

The most common nuisance organisms in anaerobic systems are the sulfate-reducing bacteria. It is generally desirable to design anaerobic operations to produce methane because it is a valuable product. If a wastewater contains high concentrations of sulfate, however, sulfate-reducing bacteria will compete for the electron donor, producing sulfide as a product. This not only reduces the amount of methane produced, but results in a product that is both dangerous and undesirable in most situations.

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