Comparative Evaluation of Sludge Reduction Routes

by Philippe Ginestet (Editor)

Overview

This project aims at the evaluation of eight various routes that potentially may allow wastewater treatment plants to produce less sludge (from 5% to 100%). It has been possible to define several routes that could be applied efficiently with knowledge of possible side-effects (risk management) and an estimation of associated costs (OPEX/CAPEX), which are quite comparable to those used in conventional sludge treatment and disposal.

Product Details

ISBN-13:	9781843391234
Publisher:	IWA Publishing
Publication date:	09/30/2006
Series:	European Water Research Series
Pages:	110
Product dimensions:	6.12(w) x 9.25(h) x 0.75(d)

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CHAPTER 1

TECHNICAL EVALUATION OF SLUDGE PRODUCTION AND REDUCTION

P.Ginestet, P.Camacho

RESULTS ON SLUDGE PRODUCTION (CONTROL)

In order to evaluate sludge reduction, it is necessary to compare the various studies on the same basis. It is hence of importance to get an overview of the sludge production on the various pilot plant investigated during this study. In order to estimate the proportion of soluble COD, it was (if value was not determined experimentally) calculated as 60% of the COD not associated to wastewater volatile suspended solids Table 1.

At least 30 control runs were carried out with activated sludge pilot plants fed with a mixture of settled and synthetic wastewater (n=5), settled wastewater (n=21) and raw wastewater (n=4). COD concentration of wastewater was in the mean of 455 ± 168 mgO2/L, of which soluble COD represented 38 ± 11 %. It is to be noted that control experiments of the oxidative route (O1 to O5) showed a proportion of soluble pollution close to experiments run with raw wastewater. Since the settling procedure was shortened (from 2 h to 15 minutes) by the partner, these experiments can be classified as raw wastewater although they were considered as settled wastewater initially. Hydraulic retention times (HRT) was set generally at 1 day, and sludge retention time was ranging from 6 to 155 days (but 70% of control experiments were run between 10 and 20 days), (Ginestet, 2001).

Looking at the sludge production resulting from control runs, a mean sludge production yield of 0.32 ± 0.08 gTSS / gCOD removed, and 0.25 ± 0.06 gVSS / gCOD removed were observed. This apparent variability was reduced if one consider these values by the nature of wastewater and for the classical range of SRT (10 to 20 days), as shown in Table 2.

Furthermore, one can draw a figure representing the sludge production as a function of the nature of wastewater (raw, settled, settled+synthetic) and the sludge retention time in the system Figure 3

The effect of the sludge retention time and wastewater quality is clear: there is a decrease of the observed yields with the sludge retention time. In addition, levels of sludge production are higher for raw wastewater than for primary settled than for the mixture of settled and synthetic wastewater.

Hence, there are three main conclusions important to notice in order to get comparable results between the routes:

- there is a variability in the observed sludge production yield. This suggests that the reference sludge production can be highly variable from site to site. It is proposed to fix sludge production yields of the reference Figure

- the criteria to identify sludge reduction shall be percentage of sludge reduction obtained during the experiments and not be the sludge production yield of the route chosen.

- it is difficult to extrapolate results obtained with settled wastewater to raw wastewater, and from the mixture to primary settled wastewater. It was decided for the further economical study to authorize extrapolation from the mixture settled + synthetic wastewater to settled wastewater and to separate results obtained for raw or settled wastewater.

RESULTS ON SLUDGE REDUCTION PRODUCTION

Sludge reduction: raw results from pilot studies

Among the various studies performed, members of the project showed up that sludge reduction from 20% to 100% were achievable. Raw results are presented in Table 4.

Table 4 show that sludge reduction was achievable. Best results were obtained for ozone treatment (up to 100%, depending of ozone dosage), especially also because results were applicable for raw wastewater. Other promising routes were identified: ultrasonic stress showed with primary settled wastewater close to 80% sludge reduction, as thermal and PEF treatment (up to 60% for both). Less sludge reduction extents could be achieved for predation enhancement, for anaerobic routes. All sludge reduction obtained could affect not only the organic fraction of sludge but also the mineral fraction of sludge, since RSP on minerals could reach up to 60%.

Thermal and PEF approaches applied to a reconstituted raw wastewater (primary settled wastewater complemented with primary sludge daily additions) showed less good results than for primary settled wastewater (30 – 50% less good results).

Surprisingly, the HPH approach did not lead to significant sludge reduction, whereas recent papers demonstrated its efficiency at 25% sludge reduction (Camacho et al., 2002).

A particular case concerns the results of partner 4 (route "Low yield induction"), who unfortunately used only synthetic wastewater for the two configurations tested up to now: chemical uncoupling and centrifuge bioreactor yielding respectively 60% and 100% sludge reduction. For these results only obtained with synthetic wastewater, the reduction of sludge production can only be applied on the "soluble" pollution entering a WWTP, representing generally 30% and 50% of raw- and settled- wastewater COD, respectively (Ginestet, 2001). In addition, no results were given in terms on reduction of minerals within the system. For further calculation, it was assumed that the reduction observed on VSS did not act on the minerals (MSS). This means that obtainable sludge reduction under real situation would be as follow Table 5.

When speaking of sludge reduction , it is necessary to identify to which system comparison can be made.

Actually, project teams worked mainly with pilot plants fed with primary settled wastewater (thermal, electrical, oxidative, predation enhancement, mechanical), although some others routes were investigated using raw wastewater (oxidative, thermal, electrical and anaerobic routes).

Since, controls were different, it was necessary to compare what is comparable. Hence, it was decided to present separately three configurations (Raw wastewater, primary settled wastewater and overall plant with primary settling).

Predictability of sludge reduction from batch tests

One underlying question from the beginning of the project was to understand the mechanisms of sludge reduction. Besides this question, one objective was to determine how it is possible to predict sludge reduction from simple batch tests. Two major tests described within (Ginestet, 2001) were proposed:

- extent of solubilisation

- extent of biomass inactivation

In order to assess the predictability of sludge reduction, we tried to gather results from partners and to draw a correspondence between these batch tests and sludge reduction observed with pilot plant experiments. The basis for fair comparison is to plot the amount of sludge which is not produced per day versus the amount of sludge solubilised per day (Equation 1). The former is the product of control sludge production times reduction of sludge production, and the latter is the product of stress feed rate (QS. . [CODsludge]) and the percentage of sludge solubilisation.

Sludge not produced per day = f (Sludge solubilised per day)

[MATHEMATICAL EXPRESSION OMITTED] Equation 1

If one consider that the sludge retention time of the control (θXcontrol) is comparable among studies, as well as the sludge COD/VSS ratio, it is possible to integrate them as constant within the α factor. The following formula can be obtained showing a possible relationship between sludge reduction and the product of stress frequency (SF) and the percentage of sludge solubilisation (Equation 2).

RSPVSS = α · SF · %solubilisation Equation 2

The same trend can be drawn with sludge inactivation.

RSPVSS = β · SF · %inactivation Equation 3

Data available from partners concerning sludge reduction and sludge solubilisation per day were gathered together in (Figure 4).

It is quite obvious from this figure that neither solubilisation nor biological inactivation can be used as parameters for the prediction of sludge reduction extent: low solubilisation (<25%) like oxidants, UH, HPH, PEF and thermal routes can lead to variable sludge reductions of 0 to 100 %. Improving solubilisation extent (thermo-chemical) to values higher than 40% do not yield an improvement in sludge reduction. Same trend is observed for biological inactivation which can be very low, whereas high sludge reduction is observed.

This global result of the project: "There is no simple batch test for predicting extent of sludge reduction" underlines the need for continuous studies with pilot plants. Batch tests measuring solubilisation and inactivation can only bring understanding in the physical and biological phenomena occurring within stress units, but do not represent complete set of tools for rapid prediction of sludge reduction. This suggests that other parameters may explain sludge reduction, one of the major concerning the "biodegradability improvement" of sludge. Namely, solubilisation does not mean necessarily that released organic matter will effectively biodegraded. In addition, biodegradability enhancement can also be performed with no or poor "solubilisation". In addition, the synergistic effect between stress application and biological reactions occurring in the aeration tank cannot be taken into account by simple batch tests.

The major conclusion of this section is that: "There is no simple batch test for predicting extent of sludge reduction". The challenge for further research in that direction is to develop a test to assess this "biodegradability enhancement of sludge".

Sludge reduction with raw wastewater

The case of "raw wastewater" is quite simple, since the reference plant (control) and the route plant are made of a simple activated sludge process. Sludge production (PX) and reduction (RSP = 1 - PXcontrol / PXroute) are obviously representative (Figure 5).

If pilot plant results were obtained on raw wastewater, one can easily take directly those results. If results were obtained on primary settled wastewater, then we made the hypothesis (minimalist) that nothing occurs on the settleable pollution when route is applied, and that the sludge reduction concerns solely the non settleable pollution. For those calculations, we made the hypothesis (generally admitted and presented by (Ginestet, 2001) that 40% of the wastewater pollution (COD) is settleable and 60% is non settleable. Hence, the following equation can be written:

RSPrawWW = RSPsettledWW · 30%

RSPrawWW = RSPsyntheticWW · 60% Equation 4

Under these conditions, the results obtained for the various routes can be calculated with either raw data, or calculated data (using Equation 4).

Results between settled and raw wastewater for thermal and PEF routes are quite different. It is necessary to take into account the fact that for those studies, For thermal and PEF treatments using reconstituted raw wastewater (settled wastewater + primary sludge), the primary sludges were already considerable biodegraded (an estimation is that 50% of the biodegradable fraction of this sludge had disappeared when sampling occurred, because, sampling occurred at the outlet of a primary settler and after a sludge residence time of 5 days at least according to operation data). This assumption is in perfect agreement with results presented by partner 9, showing that 65% of the primary sludge COD was degradable with a first order kinetic constant of 4 d-1 (Janssen et al., 2003). Hence, one can assume that the raw results were quite underestimated in comparison with a real wastewater. The assumptions for calculation used in Equation 4 appear to be quite realistic.

The estimation of sludge reduction when anaerobic routes are applied is calculated from basic assumptions concerning anaerobic digestion as described in next paragraph, page 20.

Sludge reduction with primary settled wastewater

When looking at primary settled wastewater, the first step is to compare roughly sludge production and reduction on the same basis, without taking into account the primary sludge production (Figure 6).

If pilot plant results were obtained on raw wastewater or primary settled wastewater, one can easily take directly those. If results were obtained on synthetic wastewater, it is possible to estimate that the observed sludge reduction only applied on the soluble part of the pollution of the primary settled wastewater, as described in Table 5.

Results obtained are summarized in Table 7.

It has to be noted that anaerobic route (AF or ES) could not be compared in that configuration, hence, it was removed from the table.

For the configuration where the route is applied on sludge fed with primary settled wastewater, best results were again obtained with ozone (up to 100% are achievable, although for this study we chose the value of 70%) and ultrasounds (68%). Nevertheless, CBR, thermal and pulsed electric field routes appeared to be interesting in this configuration (RSP between 40 and 60%). Chemical uncoupling, predation enhancement and HPH appeared to be less efficient (0-30%).

However, the configuration where activated sludge plant is fed with primary settled wastewater is to our knowledge almost always coupled with anaerobic digestion. Hence it is necessary to take into account the overall sludge production of the plant, that is to include the production of primary sludge and subsequent co-digestion of primary and secondary sludge.

Overall sludge reduction with primary settled wastewater but using the whole configuration

The overall sludge production of the plant includes the production of primary sludge, secondary sludge, both being digested prior to dewatering and disposal. This reference treatment train has to be compared with the sludge reduction approach applied on the activated sludge plant fed with primary settled wastewater, as described in (Figure 7).

We have already seen that the settleable pollution of the wastewater, that is the COD that will be removed in the primary settler, represents around 40% of the total wastewater COD (Ginestet, 2001). Hence, the production of primary sludge in this configuration represents a significant input.

The fate of sludge within digestion. Digester is fed with a mixture of primary and secondary biological sludge. None of the studies (excepts the ones concerning anaerobic route) dealt with sludge anaerobic digestion. According to (Janssen et al., 2003) the primary sludge COD is anaerobically biodegradable to 60% (see Table 8). Usually, it is considered that anaerobic digesters fed with both primary and secondary sludges may provide a organic sludge reduction of 45%, and no reduction of mineral. This suggest for a digester receiving 50% primary sludge and 50% biological sludge, that the biological sludge is reduced in the digester up to 30%, since 50%*60%+50%*30% = 45%. This relationship is verified by operation data from the Evry WWTP (data not shown) and also by literature data.

According to Table 8 and results from partner 9 (Janssen et all 2003), it is proposed to evaluate the performances of digester as follow (Equation 5):

[MATHEMATICAL EXPRESSION OMITTED] Equation 5

Consequently, to compare and evaluate the various routes in a configuration like the one presented in (Figure 7), the procedure is the following:

- Raw wastewater: the raw wastewater has a COD/TSS ratio of 2.4, and a percentage of VSS within TSS of 75% which is quite classical.

- Primary sludge production:

* VSS: 40% of total wastewater COD, COD/VSS of this sludge being around 1.7.

* MSS: The primary mineral sludge production is equal to 65%*25%*TSSWW (assuming 65% of settleable solids having a VSS content 75%)

- Secondary sludge production:

* VSS: 60% of COD of the wastewater will enter biological tanks with a VSS production yield YOBSVSS = 0.24 (see Table 2).

* MSS: 60% of COD of the wastewater will enter biological tanks with a MSS production yield YOBSMSS = 0.07 (see Table 2).

Using the above assumptions, we have the following mass balance (Equation 6):

[MATHEMATICAL EXPRESSION OMITTED] Equation 6

The same equations can be written for the route

[MATHEMATICAL EXPRESSION OMITTED] Equation 7

Here we assume that biological sludge digestibility is the same for conventional treatment or route application.

Hence overall reduction of sludge production for the whole plant equipped with primary settling, activated sludge (with or without route application) and digestion.

[MATHEMATICAL EXPRESSION OMITTED] Equation 8

The percentage of VSS within TSS of residual sludge may also be calculated from Equation 7.

[MATHEMATICAL EXPRESSION OMITTED] Equation 9

An example of the calculation is given in Figure 8. It shows that a 50% of sludge reduction in the activated sludge plant yields, for the overall plant a reduction of 25%, compared to the same configuration with no route application.

Applying this procedure (Equation 8) for all the studies, the results obtained with the various routes can be re-calculated as follow Table 9. the overall excess sludge reduction is equal to 100% minus the sludge production with route application (after digestion) divided by the production of conventional treatment (after digestion).

Maximal sludge reductions are obtained for ozone (34%), and ultrasounds (38%). Thermal, PEF and hydrogen peroxide showed similar results between 20 and 30%. Table 9 indicates that taking into account the whole plant, sludge reduction extents are reduced by approximately 50%.

(Continues…)

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Table of Contents

INTRODUCTION AND OBJECTIVES, XII,
Chapter 1: Technical evaluation of sludge production and reduction, 1,
Chapter 2: Environmental impact of sludge reduction routes, 16,
Chapter 3: Economical evaluation (basic hypothesis), 20,
Chapter 4: Scaling up of processes, 32,
Chapter 5: Economical comparison of the routes, 62,
GENERAL CONCLUSION AND PERSPECTIVES, 82,
BIBLIOGRAPHIC REFERENCES, 90,