Dear Jeremy,

I found the following publication entitled "The Role of Alkalinity Citizen Monitoring "  by Brian Oram to be the best answer to your question.

http://www.water-research.net/index.php/the-role-of-alkalinity-citizen-monitoring

Alkalinity is the water's capacity to resist changes in pH that would make the water more acidic.

This capacity is commonly known as "buffering capacity." For example, if you add the same weak acid solution to two vials of water - both with a pH of 7, but one with no buffering power (e.g. zero alkalinity) and the other with buffering power (e.g. an alkalinity of 50 mg/l), - the pH of the zero alkalinity water will immediately drop while the pH of the buffered water will change very little or not at all.  The pH of the buffered solution would change when the buffering capacity of the solution is overloaded.

Alkalinity refers to the capability of water to neutralize acid. This is really an expression of buffering capacity. A buffer is a solution to which an acid can be added without changing the concentration of available H+ ions (without changing the pH) appreciably. It essentially absorbs the excess H+ ions and protects the water body from fluctuations in pH. In most natural water bodies in Pennsylvania, the buffering system is carbonate-bicarbonate ( H2CO3, HCO3, and CO3).

The alkalinity of natural water is determined  by the soil and bedrock through which it passes. The main sources for natural alkalinity are rocks which contain carbonate, bicarbonate, and hydroxide compounds. Borates, silicates, and phosphates also may contribute to alkalinity. Limestone is rich in carbonates, so waters flowing through limestone regions or bedrock containing carbonates generally have high alkalinity - hence good buffering capacity. Conversely, areas rich in granites and some conglomerates and sandstones may have low alkalinity and, therefore, poor buffering capacity.

The presence of calcium carbonate or other compounds such as magnesium carbonate contribute carbonate ions to the buffering system. Alkalinity is often related to hardness because the main source of alkalinity is usually from carbonate rocks (limestone) which are mostly CaCO3. If CaCO3 actually accounts for most of the alkalinity, hardness in CaCO3 is equal to alkalinity. Since hard water contains metal carbonates (mostly CaCO3) it is high in alkalinity. Conversely, unless carbonate is associated with sodium or potassium which don't contribute to hardness, soft water usually has low alkalinity and little buffering capacity. So, generally, soft water is much more susceptible to fluctuations in pH from acid rains or acid contamination.

Hoping this will be helpful,

Rafik

The drop in pH signifies production of acid. There are many plausible explanations; several are suggested below.

1) Does the waste contain a high concentration of ammonia? Ammonia, in addition to carbonate species, will be be measured in an alkalinity titration. Ammonia will also be lost due to volatilization and will not contribute buffer capacity to the same extent as carbonate alkalinity.  Thus, ammonia ammonia will contribute to a high value for alkalinity but will not resist pH change.

2)   The COD is very high.  Under aerobic conditions volatile fatty acids will be produced that decrease the pH.

3) There is also the possibility that the initial values were incorrect and should be re-evaluated.

Thank you all. Having been told the pH drop happens naturally, I have now been told that they add hydrochloric acid ... But it was easier to go back with some research to say that it was unlikely that the initial claim was consistent.

Interesting about the contribution to measured alkalinity from high ammonia concentrations. Do you have some ideas where I can read more about that?

Liselotte,

Many environmental chemistry text books will describe the contributions of ammonia to alkalinity.  The pKa = 9.3 for NH4+ => NH3 + H+.  Alkalinity is a measure of species that react with H+; NH3 will react with H+ to form NH4+. The contribution of ammonia to alkalinity depends on the concentration of NH3 present and the relative amounts of carbonates and ammonia.

The wastewater possibly may contain certain classes of organics that may be oxidized even in anaerobic conditions, while contributing for alkalinisation. For example, nitrate driven oxidation of methanol may occur according the stoichiometry: 6NO3- + 5CH3OH → 3N2↑ + 5CO2 + 7H2O + 6OH-. If the wastewater is then aerated, the corresponding metabolic path may be inhibited, while dissolved oxygen can participate directly at the oxidation process; for the methanol case we thus could have: 2CH3OH + 3O2 → 2CO2 + 4H2O, with a tendency for acidification, rather than alkalinisation. This can be chiefly related with CO2 participation at the following equilibrium: 2H2O + CO2 ↔ H3O+ + HCO3-. Ammonia, if present, may be gradually released to the atmosphere, consumed by metabolic processes, or react to precipitate with lime, what may also contribute for decreasing the pH. Organic acids may also be generated by aerobic fermentation processes. Dissolved O2 may participate at various redox equilibria, which may further contribute for altering the pH. The pH buffering capacity of the wastewater is obviously influential. Temperature may be also be highly influential, due to its effect on O2 and CO2 solubility, on the solubility of electrolytes and organic solutes, and owing to its major influence on metabolic activity.

If there is a significant concentration of sugars are degraded by fermentacion and produce organic acids such as acetic. Producing an acidification in  biological reactor stopping the degradation of the organic matter. It is common in efluents of beberages industries or  sugar factories or dowload alkalizing for alkali washing. The practical solution is to return part of the acidified solution at baseline  and form a regulatory sulucion with fatty acids (vinegar or acetic acid) with alkalis influent improving the process.

Si hay una concentración importante de azúcares estos se degradan por fermentación y producen ácidos orgánicos como el acético. Produciendo una acidificación del reactor biológico deteniendo la degradación de la materia orgánica presente. Es común en descargas de empresas refresqueras, ingenios azucareros o que descargen tanto alcalinizantes de los lavados alcalinos. La solución práctica es retornar parte de la solución acidificada al inicio del tratamiento y formar una sulución reguladora con los ácidos grasos (vinagres o ácido acético) con los alcalis de influente mejorando el proceso.