diff --git a/dhp-workflows/dhp-aggregation/src/main/java/eu/dnetlib/dhp/collection/plugin/base/BaseCollectorIterator.java b/dhp-workflows/dhp-aggregation/src/main/java/eu/dnetlib/dhp/collection/plugin/base/BaseCollectorIterator.java
index 579ef6127..80881acba 100644
--- a/dhp-workflows/dhp-aggregation/src/main/java/eu/dnetlib/dhp/collection/plugin/base/BaseCollectorIterator.java
+++ b/dhp-workflows/dhp-aggregation/src/main/java/eu/dnetlib/dhp/collection/plugin/base/BaseCollectorIterator.java
@@ -27,7 +27,7 @@ public class BaseCollectorIterator implements Iterator<Element> {
 
 	private Object nextElement;
 
-	private final BlockingQueue<Object> queue = new LinkedBlockingQueue<>();
+	private final BlockingQueue<Object> queue = new LinkedBlockingQueue<>(20);
 
 	private static final Logger log = LoggerFactory.getLogger(BaseCollectorIterator.class);
 
@@ -78,8 +78,6 @@ public class BaseCollectorIterator implements Iterator<Element> {
 			log.error("Error processing BASE records", e);
 			report.put(e.getClass().getName(), e.getMessage());
 			throw new RuntimeException("Error processing BASE records", e);
-		} finally {
-			this.queue.add("__END__"); // I ADD A NOT ELEMENT OBJECT TO INDICATE THE END OF THE QUEUE
 		}
 	}
 
@@ -91,8 +89,6 @@ public class BaseCollectorIterator implements Iterator<Element> {
 			log.error("Error processing BASE records", e);
 			report.put(e.getClass().getName(), e.getMessage());
 			throw new RuntimeException("Error processing BASE records", e);
-		} finally {
-			this.queue.add("__END__"); // I ADD A NOT ELEMENT OBJECT TO INDICATE THE END OF THE QUEUE
 		}
 	}
 
@@ -119,13 +115,16 @@ public class BaseCollectorIterator implements Iterator<Element> {
 
 					for (final Object o : doc.selectNodes("//*[local-name()='ListRecords']/*[local-name()='record']")) {
 						if (o instanceof Element) {
-							this.queue.add(o);
+							this.queue.put(((Element) o).detach());
 							count++;
 						}
 					}
 				}
 			}
 		}
+
+		this.queue.put("__END__"); // I ADD A NOT ELEMENT OBJECT TO INDICATE THE END OF THE QUEUE
+
 		log.info("Total records (written in queue): " + count);
 	}
 
diff --git a/dhp-workflows/dhp-aggregation/src/test/java/eu/dnetlib/dhp/collection/plugin/base/BaseCollectorIteratorTest.java b/dhp-workflows/dhp-aggregation/src/test/java/eu/dnetlib/dhp/collection/plugin/base/BaseCollectorIteratorTest.java
index dfefa082f..2e4cfdd0b 100644
--- a/dhp-workflows/dhp-aggregation/src/test/java/eu/dnetlib/dhp/collection/plugin/base/BaseCollectorIteratorTest.java
+++ b/dhp-workflows/dhp-aggregation/src/test/java/eu/dnetlib/dhp/collection/plugin/base/BaseCollectorIteratorTest.java
@@ -2,6 +2,11 @@ package eu.dnetlib.dhp.collection.plugin.base;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 
+import java.util.HashMap;
+import java.util.Map;
+
+import org.apache.commons.lang3.StringUtils;
+import org.dom4j.Attribute;
 import org.dom4j.Element;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
@@ -19,11 +24,37 @@ public class BaseCollectorIteratorTest {
 
 		final BaseCollectorIterator iterator = new BaseCollectorIterator("base-sample.tar", new AggregatorReport());
 
+		final Map<String, Map<String, String>> collections = new HashMap<>();
+
 		while (iterator.hasNext()) {
 			final Element record = iterator.next();
-			// System.out.println(record.asXML());
+
 			count++;
+
+			if ((count % 1000) == 0) {
+				System.out.println("# Read records: " + count);
+			}
+
+			for (final Object o : record.selectNodes("//*[local-name() = 'collection']")) {
+				final Element n = (Element) o;
+				final String collName = n.getText().trim();
+				if (StringUtils.isNotBlank(collName) && !collections.containsKey(collName)) {
+					final Map<String, String> collAttrs = new HashMap<>();
+
+					for (final Object ao : n.attributes()) {
+						collAttrs.put(((Attribute) ao).getName(), ((Attribute) ao).getValue());
+					}
+
+					collections.put(collName, collAttrs);
+				}
+			}
 		}
+
+		collections.forEach((k, v) -> {
+			System.out.println(k);
+			v.forEach((ak, av) -> System.out.println(" - " + ak + "=" + av));
+		});
+
 		assertEquals(30000, count);
 	}