public class Multi {

    public static Set<String> parse(String values) {
        Set<String> s = new Set<String>();
        if (values != null && values != '') {
            for (String part : values.split(';')) {
                s.add(part.trim());
            }
        }
        return s;
    }

    public static String format(Set<String> values) {
        if (values == null) return null;
        List<String> l = new List<String>(values);
        l.sort();
        return format(l);
    }
}

public class MyBatchable implements Database.Batchable<SObject> {

    public Database.QueryLocator start(Database.BatchableContext context) {
        return Database.getQueryLocator([select MyField__c from MyObject__c]);
    }

    public void execute(Database.BatchableContext context, List<MyObject__c> scope) {
        for (MyObject__c sob : scope) {
            sob.MyField__c = Multi.format(Multi.parse(sob.MyField__c));
        }
        update scope;
    }

    public void finish(Database.BatchableContext context) {
    }
}