| In addition to the adaptability and controllability, the oxygen transfer capacity and the aeration efficiency characterise the performance and economy of aeration installations in activated sludge plants. In order to standardise the measurement technique for oxygen transfer tests a Guideline for the Determination of the Oxygen Transfer Capacity of Aeration Systems in Clean Water was published in 1978 [1]. In Austria, the ÖNORM (Austrian Standard Specification) M 5888 "Wastewater Treatment Plants, Oxygen Transfer Performance of Aeration Installations, Determination in Clean water" was published also in 1978 [2]. In the USA, in 1984, the American Society of Civil Engineers (ASCE), with international participation published the ASCE Standard (comparable with a Standard Specification) "Measurement of Oxygen Transfer in Clean water". Following minor amendments this was published in 1992 in a 2nd Edition [3]. Completely new is the "Standard Guideline for In-Process Oxygen Transfer Testing" [4] also elaborated by the ASCE with international participation. While both the measurement technique and the test evaluation are equal for the determination of the oxygen transfer in clean water and under process conditions (with the exception of off-gas measurements), both measurements in clean water and also measurements under process conditions are dealt with in this Advisory Leaflet. In Germany and Austria one still works with a reference temperature of T = 10° C for oxygen transfer [1,2]. As a rule the highest oxygen demand occurs with the highest reactor temperature (in ATV Standard A 131 it is, for example, recommended that the aeration system should be designed for the oxygen demand at T = 20° C). Internationally it has, for some considerable time, been usual to relate the oxygen transfer to T = 20° C [3, 4]. In this Advisory Leaflet T = 20° C is therefore also taken as the standard temperature. For conversion the following applies: OC (T=20° C) = 1.02 · OC (T=10° C) The difference between the values for T = 10° C and T = 20° C is, therefore, negligibly small. From clean water tests there is sufficient experience available on many types of aeration installation. The requirements for guarantee tests in clean water should therefore be limited to special cases; additionally, with small plants, the costs for clean water tests often bare no relation to the benefit. Much more important and interesting for the operating behaviour of aeration installations are measurements under process conditions with mixed liquor. Here, principally the maximum oxygen transfer capacity and the aeration efficiency with the current average loading are of interest. In this Advisory Leaflet, due to past experiences, unclear situations and/or the existence of different possibilities often have the note "to be agreed beforehand". This agreement must be made between the customer and the supplier of aeration facilities and, if required, also with the institution which carries out the oxygen transfer tests. This Advisory Leaflet applies only for the determination of the oxygen transfer in activated sludge plants. It does not apply for aerated wastewater lagoons. The following are to be agreed and, as far as possible, already taken into account with the request for offers: · Which water, if not drinking water, will be used for clean water tests, comp. 2.1.1. · For which aeration settings and how many tests with each aeration setting are to be carried out, comp. 3.3.1. · Which measurement technique is to be applied, comp. 2.5 · Whether measurements are to be carried out even with very low water temperatures, comp. 3.1.6. · Whether sodium sulphite should be added in dry form, comp. 3.2.2. · How the power input of the aeration installation is to be measured, comp. 3.1.4. · How the air flow rate is to be measured, comp. 3.1.5. · With what deviations are given guarantees to count as being met, comp. 5.
(New German Version available: DWA-M 209, April 2007) |