Experimental assessment of substrate storage yield

Abstract

Substrate storage under dynamic conditions is recently regarded as a significant process for activated sludge systems. The dynamic conditions and substrate gradients convey activated sludge cultures to develop a storage response when external substrate is present in the system. Substrate storage is incorporated into activated sludge modeling with Activated Sludge Model No. 3 (ASM3) and with biochemical models for pure substrates. ASM3 has been proposed for activated sludge systems both for aerobic and anoxic conditions. Introducing storage phenomena has also introduced a number of stoichiometric and kinetic coefficients making the model rather complicated with many degrees of freedom. Some default values have been proposed, but calibration of kinetic and stoichiometric parameters is needed for various applications. The storage yield $(Y_{STO})$ is one of the most important parameters of the model, since it represents the stoichiometric amount of substrate converted into storage products, which are subsequently utilized for growth. The assessment of $(Y_{STO})$ is therefore crucial for the accurate estimation of the overall electron acceptor utilization and sludge production. The information on the magnitude of the storage yield was mostly derived from pure culture studies. ASM3 suggested value of $Y_{STO}$ as 0.85 mgCOD/mgCOD under aerobic conditions for domestic sewage. An experimental procedure was developed for the respirometric determination of bacterial storage yield $(Y_{STO})$ as defined in the Activated Sludge Model No. 3. The proposed approach is based on the oxygen utilization rate (OUR) profile obtained from a batch test and correlates the area under the OUR curve to the amount of oxygen associated with substrate storage. The procedure is based on respirometry and does not involve measurement of storage products, as it will not always be possible or reliable to determine the amount of all the storage products when a complex substrate such as domestic sewage is concerned. It is theoretically possible to compute substrate storage yield $(Y_{STO})$ if the oxygen used for the storage of a known amount of readily biodegradable COD can be determined by means of respirometric measurements. Such measurements however only provide the total oxygen utilization rate (OUR) of the system and not the OUR specific for the process of interest alone. Thus, it is necessary to understand and interpret the components of a total OUR versus time curve associated with the utilization of a readily biodegradable substrate. Model simulation was used to evaluate the procedure for different initial experimental conditions. Obtained results indicated that the proposed procedure was quite consistent with model-input values, involving an error of less than 2%, aside from analytical errors associated with standard COD measurements, for tests to be conducted with feeding (F/M) ratios over 0.1 gCOD/g cellCOD. The procedure was used to determine the storage yield, $Y_{STO}$, associated with acetate, glucose and do¬mestic sewage, together with mixtures of acetate/glucose and acetate/domestic sewage at different initial F/M ratios. $Y_{STO}$ was calculated as 0.78 gCOD/gCOD for acetate, 0.87 gCOD/gCOD for glucose and 0.96 gCOD/gCOD for domestic sewage. The high $Y_{STO}$ level related to domestic sewage, consistently obtained for a wide range of initial F/M ratios, challenges the validity of the concept of $S_s$ in ASM3, which is defined as the biodegradable fraction of the soluble substrate and tested in the study. The assumption introduced with ASM3 that the entire soluble biodegradable COD could be regarded as readily biodegradable substrate, although valid for pure substrates, requires, as in this case, careful evaluation for wastewaters with more complex substrate compositions. If readily biodegradable substrate is actually less, the corresponding oxygen consumption can only be interpreted with a superficially higher $Y_{STO}$ . The experiments conducted on substrate mixtures confirmed the validity of $Y_{STO}$ values calculated for individual substrates, yielding a transient pattern reflecting the character of the dominant substrate fraction in the mixture. For glucose/acetate mixtures, they provided a clear indication of a faster storage rate for glucose as compared to acetate. The proposed procedure may also be used to calculate initial readily biodegradable COD concentration, $S_{sı}$, for a generally adopted $Y_{STO}$ value, as it defines a stoichiometric procedure between the storage yield and the available readily biodegradable substrate.

Özet

Çevre biyoteknolojisindeki son gelişmeler doğrultusunda aktif çamur sistemlerinde substrat depolama kavramı önemli bir proses olarak kabul edilmektedir. Substrat depolama kavramı, aktif çamur modellemesine, Aktif Çamur Modeli No. 3 (ASM3) ve saf substratlar için önerilen biyokimyasal modellerle dâhil edilmiştir. Depolama kavramının ortaya konması, modele çok fazla serbestlik derecesi sağlayarak, modeli daha karmaşık bir hale getiren pek çok kinetik ve stokiyometrik katsayının da eklenmesine yol açmıştır. Substrat depolama dönüşüm oranı, Ysm, ASM3 modelindeki en önemli parametrelerden biridir. Depolama dönüşüm oranı, substratın stokiyometrik olarak ne kadarının depolama ürünlerine dönüşeceğini belirleyen etkin bir model bileşenidir. Bu çalışma, ASM3 modelinde tanımlanan substrat depolama dönüşüm oranının (Ysm), deneysel olarak belirlenmesini amaçlamaktadır. Önerilen yöntem, atıksularda depolama ürünlerinin miktarlarının belirlenmesini gerektirmeyen, respirometrik bir prosedürdür. Yöntemde, model simulasyonlarından yararlanılarak, depolama prosesi için tüketilen oksijen miktarının oksijen tüketim hızı (OTH) eğrisi üzerinde belirlenmesinin esasları ortaya konmuştur. Önerilen yaklaşım kesikli deneylerle elde edilen oksijen tüketim hızına (OTH) dayanmaktadır. Yöntem substrat depolaması için kullanılan oksijen miktarının hesaplanması için grafiksel bir metot tanımlamaktadır. Yöntemin değerlendirilmesi için farklı deneysel başlangıç koşullarının kullanıldığı model simulasy onlarından yararlanılmıştır. Önerilen yöntem ile Ysm değeri %2 'den daha düşük hata oranları ile belirlenebilmiştir. Deneysel yöntem ile, asetat, glikoz ve evsel atıksuyun kullanıldığı ve farklı F/M oranlarında yürütülen respirometrik testler sonucunda, depolama dönüşüm oranları, asetat için 0.78 gKOİ/gKOİ, glikoz için 0.87 gKOİ/gKOİ ve evsel atıksu için 0.96 gKOİ/gKOİ olarak belirlenmiştir.