Alkalinity and pH

Sufficient alkalinity is essential for proper pH control. Alkalinity serves as a buffer that prevents rapid change in pH. Enzymatic activity or digester performance is influenced by pH. Acceptable enzymatic activity of acid-forming bacteria occurs above pH 5.0, but acceptable enzymatic activity of methane-forming bacteria does not occur below pH 6.2. Most anaerobic bacteria, including methane-forming bacteria, perform well within a pH range of 6.8 to 7.2.

The pH in an anaerobic digester initially will decrease with the production of volatile acids. However, as methane-forming bacteria consume the volatile acids and alkalinity is produced, the pH of the digester increases and then stabilizes. At hydraulic retention times >5 days, the methane-forming bacteria begin to rapidly consume the volatile acids.

In a properly operating anaerobic digester a pH of between 6.8 and 7.2 occurs as volatile acids are converted to methane and carbon dioxide (CO2). The pH of an anaerobic system is significantly affected by the carbon dioxide content of the biogas.

Digester stability is enhanced by a high alkalinity concentration. A decrease in alkalinity below the normal operating level has been used as an indicator of pending failure. A decrease in alkalinity can be caused by 1) an accumulation of organic acids due to the failure of methane-forming bacteria to convert the organic acids to methane, 2) a slug discharge of organic acids to the anaerobic digester, or 3) the presence of wastes that inhibit the activity of methane-forming bacteria. A decrease in alkalinity usually precedes a rapid change in pH.

The composition and concentration of the feed sludge directly influence the alkalinity of the digester. For example, large quantities of proteinaceous wastes transferred to the anaerobic digester are associated with relatively high concentrations of alkalinity. The alkalinity is the result of the release of amino groups (-NH2) and production of ammonia (NH3) as the proteinaceous wastes are degraded. Also, thickened sludges have relatively high alkalinity. This alkalinity is due to the increased feed rate of proteins within the thickened sludges.

Alkalinity is present primarily in the form of bicarbonates that are in equilibrium with carbon dioxide in the biogas at a given pH. When organic compounds are degraded, carbon dioxide is released. When amino acids and proteins are degraded, carbon dioxide and ammonia are released.

The release of carbon dioxide results in the production of carbonic acid, bicarbonate alkalinity, and carbonate alkalinity (Equation 16.1).The release of ammonia results in the production of ammonium ions (Equation 16.2).

The equilibrium between carbonic acid, bicarbonate alkalinity, and carbonate alkalinity as well as ammonia and ammonium ions is a function of digester pH (Figure 16.1). Bicarbonate alkalinity is the primary source of carbon for methane-forming bacteria.

Significant changes in alkalinity or pH are introduced in an anaerobic digester by substrate feed or the production of acidic and alkali compounds, such as organic acids and ammonium ions, respectively, during the degradation of organic compounds in the digester.

Alkalinity in an anaerobic digester also is derived from the degradation of organic-nitrogen compounds, such as amino acids and proteins, and the production of carbon dioxide from the degradation of organic compounds. When amino acids and proteins are degraded, amino groups (-NH2) are released and alkalinity is produced. When amino groups are released, ammonia is produced. The ammonia

CO2 + H2O o H2CO3 o H+ + HCO3- o H+ + CO32 NH3 + H+ o NH4+

CO2 < ----- > H2CO3 < ----- > H+ + HCO3- < ----- > H+ + CO32'

Figure 16.1

dissolves in water along with carbon dioxide to form ammonium bicarbonate (NH4HCO3) (Equation 16.3).

However, the degradation of organic compounds produces organic acids that destroy alkalinity. For example, as a result of the degradation of glucose, acetate is formed (Equation 16.4).This acid destroys alkalinity, for example, ammonium bicarbonate (Equation 16.5), and the alkalinity is not returned until methane fermentation occurs (Equation 16.6).

C6H12O6 ^ 3CH3COOH (16.4)

3CH3COOH + 3NH4HCO3 ^ 3CH4COONH4 + 3H2O + 3CO2 (16.5) 3CH3COONH4+ + 3H2O ^ 3CH4 + 3NH4HCO3 (16.6)

Although anaerobic digester efficiency is satisfactory within the pH range of 6.8 to 7.2, it is best when the pH is within the range of 7.0 to 7.2. Values of pH below 6 or above 8 are restrictive and somewhat toxic to methane-forming bacteria (Table 16.1). To maintain a stable pH, a high level of alkalinity is required.

If the feed sludge to the anaerobic digester does not contain alkali compounds or precursors of alkali compounds, alkalinity must be added to the digester to maintain stable and acceptable values for alkalinity and pH. The quantity of alkalinity to be added should be based on the anticipated organic acid production capacity of the sludge feed (1 g of volatile acids per gram of volatile solids). Also, if the rate of acid production exceeds the rate of methane production, alkalinity must be added. A higher rate of volatile acid production than methane production usually occurs during start-up, overload, loss of adequate temperature, and inhibition.

Alkalinity also may be lost or "washed out" of the digester. When increased wastewater temperature occurs, increased microbial activity within an activated sludge process occurs and buoyant sludge is usually produced. Increased pumping from the activated sludge process or thickener to the anaerobic digester occurs because of the presence of buoyant sludge. Increased pumping produces decreased digester hydraulic retention time (HRT) and "washout" of digester alkalinity.

TABLE 16.1 Optimum Growth pH of Some Methane-forming Bacteria























TABLE 16.2 Chemicals Commonly Used for Alkalinity Addition



Buffering Cation

Sodium bicarbonate



Potassium bicarbonate



Sodium carbonate (soda ash)



Potassium carbonate



Calcium carbonate (lime)



Calcium hydroxide (quick lime)



Anhydrous ammonia (gas)



Sodium nitrate



Several chemicals can be used to adjust alkalinity and pH in an anaerobic digester (Table 16.2). Because methane-forming bacteria require bicarbonate alkalinity, chemicals that release bicarbonate alkalinity directly are preferred. Of these chemicals, sodium bicarbonate and potassium bicarbonate are perhaps the best chemicals of choice because of their desirable solubility, handling, and minimal adverse impacts within the digester. For example, overdosing of these chemicals does not cause the pH of the digester to quickly rise above the optimum. Also, of all the cations released by the alkali chemicals used for alkalinity addition, sodium and potassium are the least toxic to the bacteria in the digester. Chemicals that release hydroxide alkalinity, for example, caustic soda, are not effective in maintaining proper alkalinity in the digester because of the bicarbonate alkalinity requirement of methane-forming bacteria.

Lime (CaCO3) may be used to increase digester pH to 6.4, and then either bicarbonate or carbonate salts (sodium or potassium) should be used to increase the pH to the optimum range. Lime increases pH quickly and dramatically, but lime does not significantly increase alkalinity. Overdosing with lime may easily cause the pH to exceed the optimum pH range.

Caution should be used when using hydrated lime or quick lime [calcium hydroxide (Ca(OH)2)] and soda ash [sodium carbonate (Na2CO3)] to increase alkalinity. Calcium hydroxide and sodium carbonate first react with soluble carbon dioxide in the sludge (Equations 16.7 and 16.8, respectively). If carbon dioxide is removed too rapidly or in too large a quantity from the sludge, then carbon dioxide from the biogas will replace the carbon dioxide lost from the sludge. When carbon dioxide is lost from the biogas, a partial vacuum condition develops under the digester dome. This condition may cause the digester cover to collapse. Also, as the concentration of alkalinity increases in the anaerobic digester, the continued use of quick lime results in the precipitation of calcium carbonate (Equation 16.9).

Anhydrous ammonia also may be used to adjust alkalinity and pH. Ammonia reacts with carbon dioxide and water, resulting in the production of ammonium bicarbonate (Equation 16.10). Ammonium carbonate adds alkalinity and is available to react with volatile acids, resulting in the production of volatile acid salts (Equation 16.11).

*R represents the non-carboxyl (-COOH) portion of the volatile acid.

Anhydrous ammonia also may help to dissolve scum layers. Although the addition of anhydrous ammonia has several benefits for an anaerobic digester, there are some concerns. Anhydrous ammonia may produce a negative pressure in the digester by reacting with carbon dioxide. In addition, at elevated pH values excess ammonia gas may cause toxicity.

If pH and alkalinity both must be increased in an anaerobic digester, sodium carbonate may be used to increase pH if it drops below 6.5. Sodium carbonate also replenishes alkalinity. If sodium bicarbonate, sodium carbonate, or sodium nitrate is added too rapidly to an anaerobic digester, a foaming problem may develop. Sodium bicarbonate and sodium carbonate release carbon dioxide on addition, whereas sodium nitrate releases molecular nitrogen (N2) and nitrous oxide (N2O) upon addition.

Caution also should be used when adding sodium nitrate, because the release of nitrate ions (NO3) increases the oxidation-reduction potential (ORP) of the digester. The ORP of the digester should not be allowed to increase above -300mV, for example, -250mV, because methane-forming bacteria cannot produce methane at ORP values greater than -300mV in a mixed culture.

Any chemical selected for addition to the digester should be added slowly to prevent any adverse impact on the bacteria due to rapid changes in alkalinity, pH, ionic strength, or ORP.

Caution should be exercised in the choice of the chemical used for pH/alkalin-ity adjustments. The precipitation of CaCO3 creates unwanted solids, and the large quantities of a single cation, for example, Na+, presents the potential for alkali metal toxicity. Therefore, it may be preferable to use mixtures of cations, for example, Ca2+ from Ca(OH)2, Na+ from NaOH, and K+ from KOH, for pH/alkalinity control.

Although the pH of the digester is more easily and quickly determined than the alkalinity of the digester, the pH is only an indication of what has already happened in the digester, whereas changes in alkalinity indicate what is happening in the digester. The alkalinity of the digester indicates whether alkalinity addition or corrective measures are needed.

Excessive alkalinity in the digester should be avoided. Excess alkalinity can be destroyed or neutralized with the addition of ferric chloride or citrate.

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  • torsten
    How to increase alkalinity of digester?
    4 years ago
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    How ammonia maintain the ph in anarobic tank?
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    How metane ph soda ash?
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