1 Cationic Starches as Substitutes for Synthetic Cationic

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industrial plant METHA, can be substituted successfully by cationic starches. .... dewatering experiments have shown that the synthetic cationic polymer PA can ...

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Cationic Starches as Substitutes for Synthetic Cationic Flocculants in Waste Water Treatment M. Shirzad-Semsar, W.-M. Kulicke, M. Lotz*

Institute for Technical and Macromolecular Chemistry, University of Hamburg Bundesstr. 45, D-20146 Hamburg, Germany; e-mail: [email protected] *Emsland-Stärke GmbH, Emslandstr. 58, 49824 Emlichheim ABSTRACT: A solid-liquid separation of colloidal dispersed solids in waste water treatment is difficult because of their small dimension and equal charge. To achieve flocculation, flocculants are added to waste water. So far, inorganic flocculants were used for this purpose. In more modern plants, synthetic polymeric flocculants are used nowadays. An example is the industrial plant METHA (MEchanical Treatment and Dewatering of HArbour-sediments) which already went on line in 1993. A satisfactory solid-liquid separation of the harbour silt was only possible with a complicated double dual flocculation process. However, synthetic flocculants show an unwanted fish toxicity and bad biodegradability. Therefore, it is the purpose of this contribution to check whether synthetic flocculants, which are used in the industrial plant METHA, can be substituted successfully by cationic starches.

Challenges and Results: In order to keep the waterways free, the dredged material from nearly all harbours worldwide is disposed of in the open sea. Due to the geographical location of the Hamburg harbour (it is located at a distance of approx. 80 km from the North Sea), 1.2 million m3/a of dredged material is cleaned according to the latest ecological standards at a plant called METHA (MEchanical Treatment and Dewatering of HArbour-sediments). Using a double dual flocculation process with four synthetic flocculants with different molar masses, ionogenities and concentrations, it was possible to achieve a good filterability, a good dewatering efficiency and sufficient shear strength of the flocs as well as a clean filtrate1,2. Improvements have led to a modified flocculation concept3. Now, three different synthetic flocculants are used during the flocculation process. As it is, synthetic polymers based on polyacrylamide exclusively serve as flocculants. Table 1 shows an overview of different synthetic flocculants used in the plant METHA.

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TABLE 1: Synthetic flocculants used in the plant METHA flocculant 1. cationic PA 2. anionic PAAm-AA 3. cationic PTAC

charge [%] 100 41 51

Mw[g/mol] 6.4*104 3.2*106 6.1*106

c [kg/t] 0.03 0.35 0.9-1.2

However, since synthetic flocculants possess an unwanted toxicity, it was the purpose of this investigation, in cooperation with Hamburg Port Authority, Emsland-Stärke GmbH and CUTEC Institute GmbH, to find out whether the synthetic flocculants presently used can be successively substituted by starch derivatives. This issue is especially important because starch derivatives have a lower toxicity (see Table 2)4.

TABLE 2: Maximum tolerated dose (MTD) and 100 % lethal dose (LD100) by Hens Fertile Egg-Test (HEST-Test) for cationic potato starches with different degrees of substitution (DS) and commercially applied synthetic polycations with different charge densities (LD)4. flocculant

DS

CD [mol%] -

MTD [mg/kg EM] -

LD100 [mg/kg EM] >250

cationic potato starch

0.28

cationic potato starch

0.62

-

32

166

cationic potato starch

0.95

-

10

83

synthetic flocculant PTAC

-

50

16

63

synthetic flocculant PDADMAC

-

100